You can not do alphablending ON HARDWARE (what BM does, BM has no software fallback) below 32Bit.
At 16Bit it initializes single bit alpha if alpha at all (normally 16bit is not a1r5b5g5 but a0r6g5b5), so you don't get any alpha blending anymore on the hardware side.
Having a 16Bit display enforced as well means that the hardware is seriously trash and has serious problems at working with 32Bit textures so you can not even use those parts behind the scene.
Thats why I suggested Apfelbaum or something a like as base for a software based solution. Much higher chance to be successful than trying to use "hacks" and trash like that. Games that ran on those cards were pure CPU driven actually so get a driver that does the same.
At 16Bit it initializes single bit alpha if alpha at all (normally 16bit is not a1r5b5g5 but a0r6g5b5), so you don't get any alpha blending anymore on the hardware side.
Having a 16Bit display enforced as well means that the hardware is seriously trash and has serious problems at working with 32Bit textures so you can not even use those parts behind the scene.
Thats why I suggested Apfelbaum or something a like as base for a software based solution. Much higher chance to be successful than trying to use "hacks" and trash like that. Games that ran on those cards were pure CPU driven actually so get a driver that does the same.