I don't personally know of any central repository for data on past edit-a-thons.
There might be something out there. You could probably get some information from pinging folks in CE who've worked on Project & Event Grants (Asaf Bartov, Kacie Harold) or Program Evaluation (Amanda Bittaker, Edward Galvez), or search through past grant reports... but I'm guessing the data will be sparse and inconsistent, as it is still collected in a somewhat ad-hoc fashion.
If WMF were to support the development and maintenance of standardized infrastructure for edit-a-thon tracking--something like Harsh Kothari and Jeph Paul's platform for the Indian Wikiwomen edit-a-thons (
site,
code)--this would be easier. But AFAIK that hasn't happened. If someone takes up that cause I will voice my support.
J