For funsies, I tried looking at that problem once. The only good way to do it with an existing model is with displacement maps. Of course, that depends on the polycount of the clothing item you're using.

(Poser's Morph Brush can only work on vertices it has to work with... At least, in the versions I am familiar with. With some of the newer versions, that allow for custom sub-d, I wouldn't know if Poser's Morph Tool can work in tandem with such dynamics. But, it would have to be a much heavier sub-d model than is practical if someone just wanted it so they could paint on underwear deformations in actual geometry.)

The best way to create such a displacement map would be to use an outside app that allowed you to "paint" the displacement on the model in a 3D environment. I think Blacksmith 3D allows that and many 3D modellers, even Hexagon, allow that as well.

If Poser's Morph Tool was changed, slightly, to have an option that allowed users to paint on textures/colors/maps, it'd make Poser much more handy for creating such custom displacement maps. Just a thought...