If you've got just the light with no other indications of a reverser deploying, then continue. If you've got other signs such as a control difficulty or difficulty maintaining the runway centerline, then you've already made your decision before initiating the takeoff. Reject.
If it happens at or after V1, you're going. Your calculated performance is predicated on this happening (loss of power).
Many reverse systems employ snatch-back cables which will rapidly pull one thrust lever to idle upon deployment of a reverser, depending on the type of reverser (fast enough it can break your finger or hand, too). With this in mind, while a reverser unlocked might sound like a disaster, it's hardly so, and amounts to little more than a power loss if you follow your procedure.
The decision to make is "do you have a controllable airplane?" If you're not getting secondary indications, if you're not getting directional control issues, then what you have is either a malfunctioning indication, or a problem that isn't worthy of risking a high speed rejected takeoff.
Furthermore, if your performance data has been planned using a reduced thrust takeoff, increasing thrust isn't necessary to meet the performance requirements at this stage, either...because the performance has been found to met all the applicable requirements for this departure using the value on your TOLD card or data card.
This has been the topic of a series of memos and discussion from our safety and training departments. A memo not long ago on the subject from our safety officer included some information on rejected takeoffs which may be of interest:
According to Boeing and the Flight Safety Foundation (FSF), approximately one in every 3,000 takeoffs is rejected. And, one third of those rejected takeoffs (RTOs) result in the aircraft leaving the runway. In the last 30 years, over 600 people have died as a direct result of unsuccessful RTOs. FSF claims that 80 percent of the overrun incidents could have been avoided with proper crew decisions and technique.
Although in training, the RTO is almost always initiated because of an engine failure, the large majority of RTOs on the line are not related to engine malfunctions. Boeing statistics show that only 26 percent of RTOs were for engine abnormalities, followed by 24 percent for tire/wheel failure and 13 percent for improper aircraft configuration. The remainder RTO’s were for a large variety of other reasons, such as bird strikes, improper crew coordination, ATC, etc.
Additionally, both Boeing and FSF report that the majority of aircraft involved in an RTO would have continued safely to landing had they not initiated the RTO. The number varies from 55 to 70 percent, according to the sources.
I think it's important to note that the second paragraph discusses different reasons for rejected takeoffs. It's not written to illustrate a wide vareity of reasons to reject a takeoff, but to point out that many rejected takeoffs are made for the wrong reasons, or unnecessarily.
The memo continues (with my emphasis):
Below 80 knots, Boeing recommends an RTO for: system failure(s), unusual noise or vibration, tire failure, abnormally slow acceleration, unsafe takeoff configuration warning, fire or fire warning, engine failure, or if the airplane is unsafe or unable to fly. The RTO below 80 knots should not result in runway overruns. Even at high gross weights, you should have the ability to stop on the runway.
Rejecting takeoffs above 80 knots is a completely different matter. Boeing limits the list of malfunctions worthy of an RTO between 80 knots and V1 to four conditions: fire or fire warning, engine failure, or if the airplane is unsafe or unable to fly.
Rejected takeoffs above V1 should not even be considered. The primary reason is that, all aircraft performance data is based on an RTO initiated below V1, thus making V1 your go no-go decision speed. An RTO initiated above V1 voids your computed stop margins. To further explain, if you have some condition that occurs after V1 that you believe makes the airplane unsafe or unable to fly, you will go off the end of the runway and you will substantially damage the aircraft (and possibly yourself) in the process. Again... You WILL damage the aircraft and yourself.
On our forward annunciator panel, we have a wide variety of malfunctions that can be announced. This includes loss of one of our four main hydraulic systems which provide aircraft control (we're not a 737), and engine oil pressure warnings. Company policy, with which I agree wholeheartedly, is that there are no annunciations on the forward caution panel worthy of a high speed rejected takeoff...which for us is anything above 80 knots.
Another point to consider are the limitations in continuing. I noted that performance when employing reduced thrust is predicated on completion of the takeoff with a failed engine, if one is unable to reach the point at which that occurence is planned (V1 or higher), then that concept is out the window. Further, if one has encountered a condition which makes the airplane unflyable, clearly going isn't an option...that's obvious enough it should go without saying. Limitations on power use, temperatures, speeds, etc...these are all to protect the future of the airplane...from the present until some undetermined point in time and space. However, if the future of the airplane is in jeopardy...in other words if one doesn't act to prevent the airplane from losing it's future...then the limitations are also out the window.
With THAT in mind, if you're having directional control issues with a deployed reverser and you're below V1, it's NOT the time to go pushing the power up (making directional control problems worse) and continuing the takeoff...if you can't control the airplane, then today isn't the day to fly. If you can control the airplane, then go fly get away from all those hard, dangerous objects on the ground. Handle it as an airborne emergency, and come back to a runway lined with people who wish to help you, considerably more runway ahead of you than when you had the indication at V1...and the equipment staged and ready to do the job.