If you have any docs about how to write Blitz Basic extensions I can take a look if there is any extra functionality you think you'd like, be it Protracker commands (I have the playroutines for this), compressed data loading/saving commands.
I've never actually used Blitz myself but I might have to try it.
p.s. yes sprites use the upper 15 palette entries. 3 colour sprites use different subsets of the palette though. For your information:
Sprites 0,1 use colours 17-19
Sprites 2,3 use colours 21-23
Sprites 3,4 use colours 25-27
Sprites 5,6 use colours 29-31
(colours 16,20,24,28 would be transparent but 20,24,28 are course used by 15 colour sprites)