Blitz is relatively unrestrictive but I found very little documentation regarding sprite re-use. You end up digging into the HRM and work it into the blitz commands. Otherwise there's always the in-line assembler.
It's certainly doable, I'm sure. As for the 50 frames trick - that's an excellent idea...
Currently the graphics update in 2 vbl (25fps) to keep the frame rate constant as the number of enemies increases. Updating the med player with the remaining time keeps the playback smooth, I've always thought there was a bit too much 'headroom' though.