Right, I eventually got round to rechecking byte access to vram - on
the latest rev gba and it's just like the ts4 before it - there
doesn't seem to be individual strb byte access.

Writing to odd or even addresses writes the same byte to both odd &
even pixels in the same word. So thats definately a read/modify/write
for individual pixel access. I tried a c version in gcc and it does
the same naturally.

Whats currently annoying me though are the sprite priority clashes -
where objs behind a playfield are brought above that playfield if
overlapped by an obj above the same playfield. Only fixed by
shuffling it down the oam list.. So I just hope it works with my
depth sorter :O)