A plug bonded in place could join two pipes (or hollow seat posts). Seems to me that for the stresses acting on a "kickstand" functioning foot likely being small, the form of attachment is of less concern and nearly zero safety concerns. Also I believe that the foot could well be of a smaller diameter than the rest of the lower seat post, as that portion won't need to be clamped tight (like with a seat post binder collar). This lessening of the diameter size might open up the extension foot or joining options.

Like said, for someone with access to a lathe this project would be fairly easy. I have such lathe but before I got it I developed relationships with some local machine shops for the project steps I can't do. Retired machinists and Maker Spaces are two other possible sources for this kind of work.

A wood dowl hand filed to fit and just jammed up in the post would be a quick trial and perhaps long term solution. Andy