I think those people should either participate or not. Don't just come out to comment ONCE so you can win a contest and then disappear again. As I already said, I know YLF can't control this and perhaps doesn't want to, but I think it's obnoxious.
That said, the information architect in me votes for a dedicated section of the forum or perhaps the store to house giveaways and contests. YLF has been partially commercial for a while now, so I don't see why this should bother anyone. I trust Angie and Greg would never let it get to the point where It would get out of hand or kill the spirit of YLF.
Anyway, I envision a separate section (basically a new tab that says "giveaways"), either on the forum or in the store, that should periodically stream in on the main blog, just like the store items do.