Would love to see an importer for Google Groups mailing lists too, but scraping out Groups content looks to be a bit messy.
There is a bash script for generating an mbox from a Google Group which works with a bit of fiddling (need a cookie file to get emails). It also has options for picking up new posts which helps in mirroring, though i haven’t tried it. Perhaps that would be an approach if mbox imports were rock solid and repeatable.
Also, would the mbox approach allow for importing into an existing forum? Something that might be of interest to our community is merging our existing Discourse forum, with an existing Google Group.