For clothing simulations in a seated position I used to proceed like the tutorial below that I created and which is on my site.

http://tipol-creations.e-monsite.com/en/blog/tutorial/tutoriel-poser-la-cloth-room-la-position-assise.html

With poser 13 including the "seat" object in the collision parameters gives very strange results, the clothing deforming in an exaggerated way. I managed to get around the problem by including a primitive cube object of the same size as the seat and positioned on it which I enter in the colision parameters in place of the chosen seat and which I make transparent when rendering. Do you think this poser 13 bug could be fixed?