I had a bit of time to today to read about how to get older consoles to make nicer pictures on the big screen and found that things are much simpler than I thought and this shouldn't be that hard and quite a bit less expensive than expected.
There's two parts to it. Lets assume that we can get raw analog red, green and blue signals out of the console. From this we build a SCART cable and plug it into a SCART-to-HDMI converter. That should be all thats necessary, for about $150 plus whatever shipping from the UK costs for a small box. Unless I can find a local supplier, but it would probably cost about the same.
The tricky bit is getting RGB off the console. In PAL land, the SNES and the Gamecube have RGB right on the MultiAV connector. Its good to know its there, but its not particularly useful since the MultiAV connector isn't a standard plug I can get hold of, and hacking an extra one won't help since the cable doesn't expose all the pins.
For the N64 I'll have to grab the RGB lines directly from the video DAC. It ends up being about the same amount of work because I'll be exposing a new port on the back with some kind of common connector.
So it should actually be doable, assuming the converter works. Yeah, I'm not really nutting anything out, its all documented and tested already, I just have to put all the pieces together.