There are another one: Pages using DynamicPageList (intersection-category).

"The folks at the English Wikipedia" = basically just me, by the way.

I suggest either Pppery's convention of "Pages using the N extension" or simply "Pages using N". The latter is a little more vague but still grammatically correct, given N is a capitalized proper noun, as it should be for any extension. I.e. "Pages using Score".

Some of these names are also not very precise or helpful for anyone who doesn't know what's a "WikiHiero", for example.

As a tracking category (more admin/sysadmin-oriented), I wouldn't be too concerned about it, so long as it reads well enough. "Pages using the WikiHiero extension" doesn't make sense either unless you know what an "extension" is in this context. Nonetheless, the word "extension" does help those who do know what it means, so all in all I vote for "Pages using the N extension".

Lets also not forget to document this convention at Manual:Extension registration § Tracking categories (which I just added!) and maybe also Help:Tracking categories.