Gah, what a headache. Is there some kind of tourist visa that Schengen allows? I can't believe we allow nobody to enter the Schengen area without an invite by a citizen, that just seems ridiculous. What if they just want to see the Grand Place or something? I'd say visiting FOSDEM is an even better reason for somebody who would otherwise be approved for a tourist visa anyway.
http://www.schengenvisainfo.com/tourist-schengen-visa/ indicates a letter *is* required when visiting friends and relatives but no letter is needed when no such visit is taking place (I suppose then you can't say "I'm visiting a friend" without any proof by said friend). Maybe there's a higher chance of the application being approved if you really are visiting friends/relatives, but hey, if it's possible to apply without a letter then that'll cover most people and they won't have to seek help.