Infrastructure

Would it be possible to use an arbitrary license for the published extensions at extensions.libreoffice.org?

AFAIK the code doesn't allow that at the moment; not sure this is desired either as we we'd loose the ability to perform queries based on extensions/templates licenses.

Now we can choose from a fixed list of 12 licenses (in Releases - Legal tab) which is too limiting - as I need CC0,

New licenses can be added to that list (by an admin) though, just like LibreOffice version in the compatibility section.

I have to choose a wrong license and then clarify the state in description, which can be confusing for the users.

And that'll likely cause your extension/template to be rejected upon review :-P

Adding that new license to the list would be enough for me

Added.

but it is not general and someone else might face to the same problem...

People can always use Redmine (or the “feedback” form) to request addition of new license. It's not the first time this happens :-P

My extension was accepted some time ago (actually, I think that CC0 is compatible with most of the licenses from the list), I just want to make it clearer.

Oh, probably an oversight then.