I've no doubt some old Trident puke will be along soon to tell you that they used to use reverse on number 2 engine (yawn ). It may be that the B727 used it as well - in both cases to expedite descent.
Yes you can reverse any or all engines on the 72 anytime one desires, as long of course you are at idle power. Now I do not believe that there were any procurers on reversing in flight published by Boeing, at least I never saw any. Personally I never reversed the number 2 in-flight to expedite a descent. I did reverse all 3 engines during a sim ride once when we wasting time at the end of a recurrency ride.
Instructor placed us over the outer marker of 26L at Lax, 250 Kts IAS, 10,000 feet, clean and said "Land it, straight ahead, no turns." I did, it was not easy and in no way would I ever try it in real life, unless I had no other option. Was kind of fun.