I tried a Directx10 installer on xp (rather than copying files from my Vista install).
It didn't seem to make much difference other than to have dxdiag report that I'm running 10.
Demul still doesn't give me any drop-down options for a gpu plugin. When I add it to the ini file by hand, Demul complains that it can't find it even though the path is correct.
A game that requires Dx10 just tells me that I need a newer OS.
I don't want to sink much more time into it unless I know it's possible.