Thanks everyone for the information.

My conclusion is that currently there is no compelling Ruby Wiki solution.

Instiki has too few active developers (only one, apparently?). Pimki
is similarly afflicted.

Though I greatly prefer Ruby as a technology, I can't commit to a
system that has such a tiny developer - and accordingly also user -
communities.

It's a shame that there is no attractive Ruby Wiki. I guess the reason
is that PHP already had several mature, advanced Wikis (as well as
CMSes, BBs..), with large communities of active developers, when Ruby
started to become popular.

My next step is to investigate non-Ruby Wikis, with Dokuwiki being the
first, as it was recommended by James and seems to fit my (rather
humble :-) requirements.

-Alder