I've checked the menalto2piwigo importer, as you suggest, but I think I failed to convey my point:

I'm looking for a more generic gallery exchange mechanism (my gallery was not menalto gallery 2 or 3). A mechanism where one can use standard command line tools to extract, convert, and migrate galleries (that would more or less eliminate the need for X2piwigo and Y2piwigo and Z2piwigo plugins).

One such mechanism could be representing Albums as directories and media as files in those directories, each accompanied by a separate meta-data file in simple text format. Piwigo could then deliver/require meta-data as line-based field-value-pairs: e.g. Caption=..., Description=..., ...
If Piwigo could export in the same way, then backup and replication, or other integration could be done without knowing PHP or the internal workings of Piwigo. (Of course, more advanced things would still be harder to migrate).

I guess such is not readily available.

What would be the simplest approach for me to get such a tool running (apart from getting someone else to write it of course :-) (wink wink))? Any sufficiently simple plugin I could base it on? Something even simpler than menalto2piwigo?