1) Is there an 'official' (or suggested) place in these fora where to put user-contributed plug-ins for the other to download?
We set up a new section for that
Didn't even expect a plugin so quickly ![]()
2) Are there guide lines for suggested / preferred / requested physical details of distribution? Like: one single archive, please OR multiple files are also ok; for archives, please stick to .zip (or whatever) OR .tar.gz is fine too. And so on.
Basically it's up to you, depending on your plugin. Only exception: No .exe file or something like that (so most preferably a zip), but we will set up some guidelines for that.
3) Is a specific form of licensing (or a range of roughly similar equivalent licences) suggested / required? Is the publication of sources a requirement or the simple .jar is already enough (as long as it works, of course)?
That's also up to you, it's your work. Only restriction: Users and servers should be allowed to use your plugin for free
About the source files: You don't have to publish them necessarily, so the jar file is sufficient.