It sounds like a great idea....although I love anything marshmallow! Not sure about your area, but I noticed in my local grocery store, they are now carrying a rectangular shaped, semi-flat marshmallow specifically for s'more building! It's the "traditional"/national marshmallow brand.....keep an eye out for those!
As a huuuuge smore fan myself, I'd absolutely love the idea and now I'm jealous I can't do something like that. I'd just consider it a late night snack , tho, and not a favor. Again, as someone said above, actual favors aren't necessary, people will enjoy the smores :-)
Re: favors
Planning Bio
Philly Siggy Challenge: The Ring!
http://www.walmart.com/ip/Jet-Puffed-Stacker-Mallows-Marshmallows-8-oz/16554941
You know that you don't have to have favors, right? If the s'mores eat up your favor budget, just skip favors. Winning!