You have a few choices :
- If you're targeting Maemo/Meego only, now it's the time time to think if you should make that UI in Qt. A few 2D games have been developed in QML already.
- Professional games usually have their own widget "toolkit" for SDL/plain framebuffer and they change it for every game so that the widgets are themed in the game's style. This is what most complex/3D iPhone games, WebOS games, etc. do.
- You can use one of those SDL-based widget toolkits around (many of them but I have no experience with any).
- You can code a Gtk+ or Qt launcher application, where you can configure the SDL game before launching it. This is what the built-in N900 games and DrNokSnes do (so you can try the result!).