Of course SDL is available PPC-natively for MorphOS. Gabriele Greco offers both an AmigaOS and a MorphOS version on his homepage and it's also on Aminet.

I have noticed some problems with that one and MorphOS, though. For example, under some conditions, there are wrong colors displayed on little endian graphics modes. I have tried to correct this and improve some stuff. If you port something and have color/speed probs you could try my modified version.

It's unlikely, though, that it'll be merged into the official one since the author has some different views on things. For example, I find it unacceptable that SDL mercilessly overdraws other windows when run in window mode (with certain SDL flags set at least). The author refused to change this. So I built my own SDL.

I should say, though, that this applies to some time way way back. I dunno if the MorphOS SDL version significantly improved - I do not check any Amiga newssites or mailing lists besides MorphOS ones anymore.