Gosh. Questions in response to questions. I'm not the expert. You are, aren't you?
I realise you don't want to say it, but you're implicitly saying that the only option is to continue rolling down the runway with the thrust levers retarded, and look ahead and crash visually. I'm merely testing that position.
Certified 'big jet' engines these days must go from not much thrust to 95% rated thrust in 5 seconds or less: FAR 33.73(d):
(b) From the fixed minimum flight idle power lever position when provided, or if not provided, from not more than 15 percent of the rated takeoff power or thrust available to 95 percent rated takeoff power or thrust in not over 5 seconds. The 5-second power or thrust response must occur from a stabilized static condition using only the bleed air and accessories loads necessary to run the engine. This takeoff rating is specified by the applicant and need not include thrust augmentation.
In the conditions assumed in my scenario, will the engines really have spooled down to idle in the period between you initiating braking and retarding the thrust levers, and realising the brakes aren't working?
You've initiated braking and retarded the throttles at or before V1. Let's say 2 seconds later you realise the brakes aren't working.
The good engine/s didn't spool down to idle in those 2 seconds.
Let's say another second to advance the the thrust levers.
Note the 3 second margin that's built into the V1 certification data.
Are you still going to leave the thrust levers retarded and just look ahead and crash visually?