Why not run it on WMF Cloud? In code joins will very likely work there and Cloud is supported. You are effectively asking to also support a second way here.

And I think it is by no means "needless" to say that it would be "grossly impractical" to do this in the bot code. Joining page titles from commons and en.wp could for example be done by having the DB servers return a sorted stream of page titles (incrementally with limit). Your app code would advance the stream that has the lexicographically smaller title (returning equal titles). This would require little effort on the Pi, however it would transfer two big data streams (2-3 GB maybe - so feasibility would depend on your connection, but again this would be cheap on Cloud!).