I don't believe this is currently a feature in PieFed, but I don't see why it couldn't be really. Just so long as you are aware that any rss feed made by PieFed is basically public. We can stick a long random string in the url, but there isn't any authorization built into the rss that I am aware of.
If Piefed doesn’t provide this today, is there somewhere I can request it?
You can make an issue over on the PieFed codeberg repository.