... above Vmu and airborne is there a problem with Vma with increasing thrust?
There may or may not be. If you are of the persuasion that increasing thrust during an OEI experience is good, then you need to know the following - either
(a) TOGA, on your aircraft, only runs the thrust up to the derate value .. in which case, that might be fine provided that the increase from derate/flex to derate is slowish and smooth ... or
(b) if you are going to run up above the derate, either via TOGA or Mk 1 fistful of throttles, then you MUST know what the full rated minimum speeds are and respect them. You can get a handle on this by consulting the RTOW charts for full and derate thrust pre-takeoff.
.. if you're having difficulty controlling the aircraft at the time ..
for a low weight, speed limiting takeoff, I would think that one should anticipate having an interesting ride .. if you haven't looked at this in a reasonably realistic box first .. then things might get out of hand very quickly. I used to insist on first endorsements on Type, especially initial commands, being exposed to rated thrust, minV1, aft CG, etc., OEI failures ... after the first few exercises with the usual departures, the guys would get their acts together and get on top of the stick and rudder aspects of the problem .. like anything else it all comes with practice. However, not something to be attempted for real without any prior exposure ..