Well, just tried this and it works. Lemmings is playable. But the controls get messed up if you click in the offscreen area (the black bars) and you need to restart UAE4All.
In order to move the displayed area in Lemmings you can click on the minimap in-game so there's no danger of dragging the mouse "offsides". As Lemmings has one of these X-shaped mouse-cursors in game you need to click about 5 pixels up/left of a Lemming to use him...
Also using touchscreen + dpad/nubs to move the mouse simultaneously doesn't work well.
I could make it so that "mouse mode" is for touchscreen and if you screw something up so that it becomes "decalibrated" you can switch to joystick mode and back again to reset the mouse position.
So in joystick mode you'd use the nubs as mouse and in mouse mode (=> touchscreen mode) you'd use the stylus (and the dpad maybe arrow keys now instead of mouse-movement?).
What do you think?