Esme Povirk (@madewokherd) commented about loader/wine.inf.in:
[FakeDllsWin64] 10,twain_64,sane.ds 10,twain_64,gphoto2.ds +10,,twaindsm.dll,twain_32.dll
Seems like according to the spec (Operating System Dependencies / Developing for Windows), twaindsm.dll should be in the system directory for the appropriate architecture. This would, if I understand correctly, put it in the Windows directory.
I think it would make sense to build it separately from twain_32 for this reason - the default installation behavior would then be what we want.