T. J. Brumfield

Tons of other SDL projects use opensource upscaling filters in games and emulators. Here is an example from ScummVM using SDL and nearest-neighbor to upscale 4x. https://drive.google.com/file/d/1wk87f2r4VHwYb0PrpBj9XIFS4QlbE1QM/view

There are lots of ways of doing upscaling, including OpenGL filters, but it can be done with SDL as well.

This SVN build is supposedly just the SVN build plus save states, so a diff between it and vanilla SVN should provide a recent version of a save state patch....

This may be a really ignorant statement, but don't you just need to save the machine state? CPU type speed, amount of RAM, and what is in RAM, etc?