The current sdl12 code has a compatibility layer for win32 api, but has anyone worked on a port for win32s? There is recent, extensive work to build at least one win32s "emulator" which actually maps the win32s api instruction set to a modern system api (http://stephan.win31.de/w32slist.htm and https://en.wikipedia.org/wiki/Win32s). I haven't confirmed whether this particular project overlaps with Wine since it is currently closed source. Another advantage is that win32s applications would run in dosbox since it supports Win 3.1; however, Win32s "emulation" would be freed from the constraints of translating x86 instructions and could instead directly translate the higher win32s api to a host system.
Since win32s seems to use some subset of GDI function, it would be interesting to first port the sdl12 windib driver so that it is compatible with win32s. There is also recent work on openwatcom to build win32s applications so that this is possible.