I've been making a bit more progress on this, I think it's almost 'finished' now. As in finished for a 'bit of a hack'
It will support the following: Events, Graphics, Sound and Threading
Will
not support: Timers (too slow and crappy) or the SDL mixer (too hard, see above)
As some of you know, I've been using it to help port Descent Rebirth to AGA and that game seems to be holding together ok so far. As another test, I tried to build DosBox v0.74 with my library and the sound and graphics were fine but I did find some bugs in my library with the keyboard handling (which I've now fixed). It was quite a nice test actually because it only took 10 mins of updates to go from the existing RTG SDL to a working DosBox using my AGA version
Next up I'm probably going to have a little go with an Exult port to help test my SDL library further.
I'm now thinking that if I were take the RTG graphics code from the existing SDL library and then copy that code into my version it would give us 2 fast SDL versions for 68k ports