[Edit: Ignore this post in loo of what was writen above]
I'll be doing what your doing, I just bought my 12" Referb today. Back when I was Eniregnat (a dyslexic tangerinE iBook user), the Insomnia plug-in was popular. The fear was that the life of the processor would be shortened if people ran the computer wile closed. From the posts I read over the years, running the computer wile closed never did any damage.
The PB's are different. The 12" runs the coolest of the 3 models, but it's still pretty hot to me. It is almost uncomfortably hot, even when open. I would not recommend it, even if you find a way to do it. In short, get a
iCurve , or something to help with air circulation, and to give yourself a place to stoe the keyboard. I hope you had the cash for a
bluetooth keyboard and mouse.