Options:
1) You might want to look into AccuRev's GitCentric interface. It will allow you to work in Git and have your content show up in AccuRev, or vice versa. This doesn't directly answer your question, but provides you a way to work in a tool you're familiar with.
2) If you're unfamiliar with AccuRev, they do sponsor monthly end user training sessions for free. You can sign up for the Online Instructor-led Training. The AccuRev commands you'll need most often are: keep, promote, update and merge. Again not a direct answer, but learning is a life time goal or struggle depending on your prospective.
3) Using .acignore files to keep the Git files out of AccuRev would work. The .acignore files are designed to allow you to keep files not being managed by AccuRev, noted as "(external)", from being acted upon.
You said "I’m wondering if it would be a good idea to use git “internally” - meaning I would use git locally, then doing the “official” commits with Accurev." That's the joy and freedom of being a developer... you certainly can do that if you choose. You'll need to be careful as Git will not be the official source of record, but instead AccuRev has that role. You may find yourself having to do many merges if you don't keep your AccuRev workspace current via the update command.
If you do use GitCentric, then AccuRev will be the offical source of record at your site, but you'll be able to use Git for all of your work.
Please do respond with your path forward as I'm sure there are many curious developers out there who may want to know.
David