uuh - i am not sure if one should open that can of worms. what is the
purpose of wikimania? is it to raise money? for sure not. there are
more intelligent ways to do that. is wikimania a business, i.e. the
organizer want to get as much money out of the conference as possible?
for sure not. there are other conference topics which are more
promising.

If there is interest by participants to chip in on scholarships for others to attend, I'm not sure providing an outlet for that interest means opening any cans, full of worms or other critters. It's an efficient matching of donor interest to funding needed.

Quite! I can certainly see there being people (probably some Wikimedians) who would be interested in making a donation to help others (certainly Wikimedians) to get to Wikimania. Much more interested than they are in giving to WMF's general funds to pay for cups of coffee for developers. Of course, this might result in very few donations and not be worth the effort, but you don't know until you've tried it.

By contrast I'd guess that very few banner-campaign donors are interested in Wikimania at all.

Chris