Goddard/Jon Irenicus did some work on the data side of things, but that's about it. It's not just about replacing the graphics, but a lot of the structures need their offsets scaled, coordinates used in scripts and dialogs the same and maybe more.
It's mostly a data mod if you pick a fixed upscaling factor; engine changes could just speed up some things, provide shortcuts. And there are probably some things that still need to be unhardcoded, but other than that, I imagine it could work without touching the engine.
A lot of work either way, but I'm sure there'll be enough interest and volunteers eventually.