>could p9p possibly compile and run under Cygwin?

Might be an easier port... MinGW might be preferable, as drawterm uses it, and again might be easier...
Neither would work out of the box, though.

>An extra of indirection may not be the most efficient thing, but...

My approach thus far has been to just grab the bits sam needs and compile it static...
It'd be kind of goofy to need -two- "compatibility" layers just to run an editor, but you're right in that doing so may be the only feasible way.