I've tried the double-tap, but found it too unreliable and went back to using the buttons. It's still very much playable with the buttons (although I can't figure out how to move the bike in the combat scenes) but it would be more convenient if flipped.
Anyway, if you can't, you can't, but if you do come across an sdl_flip() command or something along those lines, it would be a good feature to have in a future version of the port.
I played with the double tap all the time, well, maybe it's a matter of taste
To navigate your bike you should put the cursor to be touching either left or right screen boundary. You move your stylus either to the extreme left or to extreme right, only then the bike will be steering. To cancel the effect, tap the screen in the middle, so the cursor is no longer near the boundary. It is possible to complete all the fighting in the canyon solely on the Zaurus, I've managed to do it. I haven't tried the racing episode though, as I rememered it was quite hard even on the regular PC... Well, in case it is impossible on the Zaurus, I've heard there is some cheat in the game to bypass it, don't really know about it.
ScummVM doesn't use the SDL_Flip() function, it updates the screen only where necessary. I recall some dirty region lists maintained... Looking now, the main process seems be happening in the update_screen() function, in backends/sdl/sdl.cpp, but I don't really want to grok in all the subtleties in order to perform the rotation. Well, patches are always welcome, and the thing is perfectly hackable on the regular PC