I have an application where the user can select different outfits for their 2D animated characters. For one Poser figure, I'd like to be able to render for example different shirts. The normal alpha channel in a Poser render makes a partial mask for the shirt, but I also need a mask for the areas that the figure itself masks such as the figures neck area masking the back of the shirt when the neck shows through a v-neck for example. 

I've tried rendering the figure green or black to try to make a green screen or alpha mask, but there are issues such as anti-aliasing to green means part of the shirt edge picks up some green.

Anybody have any ideas on how to render something like a shirt so that it is complete masked and able to be placed over a base figure?

Two stage process.

Render with the figure as you did above, but then render again with figure coloured roughly the same as the shirt.  Put both renders into the same Photoshop (or whatever) document.  Use the greenscreened render to make your selection, but do the actual cutting on the second.  Since the figure was more or less the same colour as the shirt, antialiasing discolouration should be minimal.

Or use the excelent script from Semidieu "Advanced Render Settings", or only the alpha part "Render Passes4 - Alphas" :)

http://www.runtimedna.com/Advanced-Render-Settings.html
http://www.runtimedna.com/Utilities-Vol7-Render-Passes-4-Alphas.html

 Thanks for the responses. I'll give them a go. The second script looks especially promising.