I've occasionally seen portholes on ebay being flogged as collectibles but they usually want $100's for them.
If you're adventurous there is an example at
http://lerch.no-ip.com/atm/default.new.htm on how to cut blanks from 3/4 inch plate glass. Just got to find a source of cheap thick glass.
I'd hazard a guess that an issue with glass laminates might be whether they'd retains the mirror's figure over a range of temperatures. If materials had different expansion coefficients it may distort the mirror. Also couldn't out-gassing from the glues cause problems when coating the mirror?