The descriptions from both the secret characters that you get from collecting all the gold available on either campaign says they're from Enclave "Classic". Is that just an in-joke or was there some older Enclave game, or older version of it that lay in development hell until this one came out?

As far as i can tell, the Enclace was a new IP (intellectual property), and it has no "older version". I did a little research and i found nothing. Some (or all) characters can be a reference to other RPGs (both table and pc), but that's all. No previous pc games, not even board ones.
There was a planned sequel which got cancelled, and there will be a Wii-only sequel - and that's the all things connecting to the Enclave (the pc game).
After looking through some forums: some other people noticed this too, but non of them knew what is "Enclave Classic". So it's look like just a bug/easter egg... Who knows.

I tried emailing their (Starbreeze's) fanmail address with the question, but immediately got a Postmaster notice of failure. The box is either full from lots of letters about Syndicate, or from not being answered in a long time.

OK, after another bit of research i have found out... Nothing important.
Sorry, it's looks like this is the first (and probably last) of the Enclave "series". They wanted a sequel (cancelled), they wanted a Wii port in 2007(?), which they moved to 2011 (it will be 2012 i think...). And that's all.
I searched for the names of the plot characters, and nothing (OK, Vatar is also in LotR, but it's not the same one). The name of the kingdom you defend/destroy is gave me the same.
As for me, i can't do nothing more (Google failed me... Or it was just my googling skill?). But man, you asked a good question, and i want to know the answer :)
So my (our) only hope is that they will answer your e-mail.

Have a good luck with that.

As far as I recall, Enclave started out as a Quake3 style Deathmatch game. The two unlockable characters you receive for collecting all the gold are from this prototype. There are also a number of modern/industrial styled textures left over within the games files, as well as a pair of the original Deathmatch levels which were included as examples in the SDK. (Some content from one of these levels was also reused in 'The Sanctuary' from the Light campaign)