Hi,

I created a temporary material for a thin band of polygons around the waist and hems and then, in the cloth room, I set this as a constrained group. This held the mesh up whilst the shrink fitting simulation ran.

In more detail:

I use Wings3D where I can so this workflow is for Wings and Poser and Wordpad

1/
Since my shorts mesh has groups I opened the mesh in Wordpad and replaced the "g " entries with "usemtl " statements, so  "g hip2" became "usemtl hip2" and so on, and then saved this as "nogroups.obj". 

2/
I imported "nogroups.obj" into Wingsand created a new material in Wings called "constrained" the name is not important but it helps me to remember what it was for if I use the model again in six months ;-) 

3/
Still in Wings I selected an polygon edge on the waist, and on the hems and used 'edge loop' to have Wings extend that to to a circle of edges on the waist and hems.

4/
In wings I then selected polygon mode, so my loops of edges became bands of polygons.

5/
In Wings I selected material and assigned my new "contrained" material from 2 above, to these selected polygons.

6/
I selected the whole mesh and scaled it down to 50% in both X and Z. This is so that it will be smaller than its final size and an expanding Antonia will fill it out. I didnt scale it down in the Y direction because I think that would give a 'camel toe'  effect that I didn't want. 50% was just a guess by the way and an easy number to work with.

7/
I exported the mesh from Wings and Into Poser as a prop, giving it a new name 

8/
I loaded Antonia and reduced her  down to 50% in the X and Z directions and saved a keyframe.

9/
I expanded Antiona to full size and saved another keyframe, I think this was about keyframe 11.

10/
Off into Posers clothroom and I clothified the shorts and set them to collide agains Antonias  hip2 and Thighs. I also set the 'Offset' to almost zero since I wanted the shorts to shrink fit really close to Antonia's mesh.

11/
I selected material in constrained groups, and then selected the "constrained" material I had created earlier in Wings. The polygons in the constrained group will stick like glue and hold the mesh up through the simulation.

12/
I ran the simulation and Antonia nicely expanded and stretched the shorts to skin tight fit.

13/
I exported the shorts from Poser, but now I needed to get rid of the "constrained" material and restore the groups .... so back into Wings.

14/
In Wings I selected the "constrained" material in the waist and changed to "hip2" material ( you recall I changed my groups into materials in step 1 ).  I also selected the "constrained" material in the hems and changed them to rThigh and lThigh respectively.  This is easier if you change the colour of the "constrained" material in Wings to something bright, so you can easily see which polygons have that material.

15/
I exported the mesh from Wings and into Wordpad, and then replaced the "usemtl " statements with "g " statements, and that restored my groups. Note that Wings may also have created groups from the materials itself, but these can be deleted without harm.

Phew
Finally we should have a skin tight version of the mesh which has the original vertex order and retains the original texture mapping. 

Sorry if that is a bit drawn out but I thought it might be useful. I use the switch between group and material quite a lot with Wings because Wings seems to cut up meshes which have groups. Of course it can get complicated if you have material zones in your mesh as well as groups, but I didn't have any material zones in my original mesh.

Cheers

Ian