I did try this, but for whatever reason it kept getting added back automatically. I forget the details of exactly why it was happening because it was close to a year ago, and in the compatibility guide it says this is supported, but I'm not sure if it was at the time or I was running into something different. This was a contract gig for me where I knew it would be ending within a month or so, which meant I didn't bother spending a ton of time trying to figure out a long-term solution.
Interesting, I tried on a recent version so who knows? I do find that sometimes jj is a bit precious with ignored files if the file exists before the ignore rule, even if you untrack. In those situations I almost always "delete/recreate" after the rule is added.