I've enabled TextChas on the DRI Wiki. It is now necessary to answer a question when creating an user account and editing pages.
Two weeks have passed, and there was no new spam since TextChas were enabled, which is really nice for a change.
I'm sure spammers can try harder and beat TextChas if they are up to the challenge. But I definitely think that TextChas is a simple and effective solution for the current spam in freedesktop.org wikis. It will, at the very lest, allow us to fight spam against spammers more evenly.