One way to sidestep it would be if your experiment is with a community structure that's much more restrictive than Wikipedia's. If, for example, only approved editors can edit articles, then you avoid most of the spam problem. But it's not clear to me that insufficient barriers to editing are the main problem with Wikipedia.
Reasons I think it is: it's very domain-specific, which makes it possible for qualified editors to flock to appropriate domains; there is a separate command structure per Wikia, enabling access on a per-qualification basis; it practically begs you to copy content from Wikipedia as a base; it already has a financial support system in place with the ads, and presumably has solid infrastructure backing it.
I can see how some of those strengths would double as weaknesses, though.