I could fit a long thin strip of WBP ply over the gap along the length of
the back wall so that it over hangs across the tray by half an inch or so,
then I could seal / stick it to the walls / the shower tray with no more
nails or silicone or something where the ply meets the tray, then tile
over
the ply and seal with grout.

Should be fine. Plan the tiling so that you don't rely on sealant/grout for
the majority of the sealing. Basically put the "horizontal" tiles down first
and then tile down the wall to them, so that water running down the wall
hits the horizontal surface. Obviously, you want a good fall to the
horizontal surface. 45 degrees is good. ;-)

Also, I'd use some screws in there. I'd cut some triangular brackets from
some pine, screw these to the wall and screw the WBP to these.

Christian.