ASL 2.0 would work but LGPL would be preferred mainly because the rest of the codebase is LGPL.

And yes, the idea would be that the new module would only be moving forward. Technically we could add it to 5.2, but I'd really only "announce it" for whatever comes after 5.2 (whether that be 5.3 or 6.0 is still to be decided in the next few months).

Since the code already exists in Vibur (albeit in form that needs some tweaking for 5.2) as Sanne points out the actual work should be pretty trivial. I really comes down to the legalese as Sanne also mentions.