I don’t think this would fit with the forum’s current guidelines which note (in the General Category):
We generally unlist and close threads that mainly focus on promoting iNat projects or initiatives or other threads that don’t really focus on starting a conversation (like announcements, requests for surveys, a single link to a webpage without context, etc.). That said I know that there are threads that discuss some more organic identifying initiatives (IdentiFriday, etc.). You could make a post asking about whether there might be a way to do this in Forum Feedback (since this thread is really focus on iNat’s first official ID-a-thon)?
You could also use other routes to promote a user-organized ID-a-thon like projects, journal posts, iNat Discord, etc.