The only true precedent we have here is the public Collection server run by PediaPress, which is a wonderful service used by several hundreds wikis for years. It ensures a comparatively smooth installation of the Collection extension even for the most niche formats. https://www.mediawiki.org/wiki/Extension:Collection#Generating_PDFs.2C_OpenD...
One could perhaps also consider DNSBL services by SpamHaus and others. https://www.mediawiki.org/wiki/Manual:Combating_spam#DNSBL But I don't know how many are using them and they're not really effective, so we don't really have a service comparable to Akismet. The "Antispam" extension claims to be one, but at the moment it's used by 3 wikis only.
Nemo