Try this?
Put your figure and undies in Poser, put them in wireframe mode, and take a screengrab from the front, side, and rear camera.

Then put the dress on the figure, put  them in wireframe mode, and take the screenrgabs.

You should now have a pretty good idea, where to "paint" your undies on the dress UV-Map.
Look ate the polygon intersections and work from there.

good luck

 

Poser 1, 2, 3, 4, 5, 7, P8 and PPro2010, P9 and PP2012, P10 and PP2014 Game Dev
"Do not drive faster then your angel can fly"!