"They only want poser renders in the poser gallery, but where do you put a poser-heavy carrara render?"

Anytime you use more than one application, you are welcome and sometimes encouraged to post it in the Mixed Medium gallery.  That's what it's there for.  You'll see quite a few Poser-heavy images there (among other things). 

And on a side note: There's also a gallery specifically for folks who what to display their modeling skills.  Check out the 3D Modeling gallery sometime. 

Irene-