I don’t know of such an alternative. A quick solution would be to use something like GeoNotes to take geolocated notes.
As far as a self-hosted solution goes, I’d just like to point out that you wouldn’t need a self-hosted database of places. You could query Ouverture (or Google, OSM, etc.) for places near you, and you’d just need to store the check-in on your server with a basic API. This is an interesting problem, and not super hard to implement.
Jiminy Cricket, imagine not being able to tell the difference between an error code and an image of an error code, and imagine subsequently, for some reason, not immediately inspecting the HTTP request and response. Sounds like a very real #programming #devops problem.