Colloquially, this is definitely meta, but whether it is in stackexchange technical terms, I don't have an opinion on.
As for the actual question, I'd say it's "WordPresses".
I have never given this consideration when it comes to WordPress, but if you'd check "press (noun)" in a dictionary, it should give you "presses" as a plural. And if that's the case, it goes for all nominal composita that end in "press" as well, such as "bench press" and "bench presses".
Needing to use the plural can obviously be avoided by paraphrasing it as "several installations of WordPress", or the like, which I personally feel sounds much nicer anyway.
Oxford Dictionary Online (British) as well as my paperback Merriam Webster (US) say the same:
If the noun ends with -ch, -s, -sh, -x, or -z, add -es to form the plural
bus --> buses
kiss --> kisses
[update (pertaining to the OP's update)]
If you were to describe something that is inherent to WordPress, say the core or some API, then you could call it "WordPress's core" (weird, but grammatically correct), i.e. "the core that belongs to WordPress" (Stephen's ownership, or "possessive", case).
Your usage is wrong though, with or without (without anyway) an s following the apostrophe, because you mean to say "projects involving WordPress" and not "projects belonging to WordPress". In this case though, life is even simpler than you made it for yourself: "projects" is what needs to be in plural form. In that case, "WordPress" simply remains "WordPress".
As per English Language & Usage:
Whether there is a plural form depends entirely on whether there is actually a singular form.
In the case of WordPress, there isn't a singular form. You don't say “I implemented my blog as a WordPress.” It’s using WordPress or even on WordPress or in WordPress, but not as a WordPress.
Consequently there is no plural form.
This doesn't apply to all trademarked names, though. One may very well talk about Compaqs or Pepsis or Hoovers or even Guinnesses and Tumblrs. All of these have a singular form — a Compaq [computer]; a [drink made by] Pepsi; a [vacuum cleaner made by] Hoover; a [proprietary drink made by] Guinness; a [blog built on] Tumblr.
Additionally, it probably depends to some extent on how euphonious the plural form (if there might be one) actually is. Even though one might talk of a Kleenex for a tissue, a few Kleenexes is unlikely to occur. WordPresses may well fall into that category as well as the “no plural” category, even if Guinnesses does not.