Certainly it's trivial to allow anonymous posting of comments - on my website (
http://mark.aufflick.com/) you can make a general comment without logging in - I just removed the requirement to be uid != 0 on the relevant GC pages, and from memory I had to manually add an entry for UID 0 in users (or persons - one of the two) although i believe this is done by default in 5.0? (My site is based on 4.6.?)
I don't see why the same system couldn't work for posting. If the UID is 0 place a large red suggestion that they login (a nice drawcard would be to get added functionality like editing their posts - like logged in users can edit their general comments) and then just let them post anyway even if they chose not to register.
