Is there any advantage to running mplayer using vo and ao as sdl? Or will this be slower than the direct framebuffer writes that mplayer currently does?

One advantage would be no corruption of the display like you get when both mplayer and Qtopia are writing to the framebuffer at the same time.

It certainly would be interesting to see the performance.

But perhaps someone with a bit of time on their hands can write something similar to MPlayerShell in native C++ so that it can create a blank full screen while playing and controlling mplayer and intergrate better with Qtopia's file associations etc.