So 4 years on I've started working on Version 3 of the S2DL libraries. Version 3 will switch completely to SDL2 but will try and do as few changes to the S2DL design as possible. So all OpenGL calls are being removed and all hardware acceleration left to SDL2.

So far I have been able to convert
S2DLDraws (most of)
S2DLImage (most of)
S2DLFont - changed 3 lines of code
S2DLInput - Added 12 new constants....

The 2 basic demos are working
HelloWorld (Simple app to show text on the screen)
Balls (Simple game showing sprites - not animated - and mouse input)

Will try finish what I've done and make it available again.