I went back and reviewed that thread on the tagging list. Using ref:usfs is mentioned several times, and even suggested it's usage: https://lists.openstreetmap.org/pipermail/tagging/2019-August/047463.html

However using a semi-colon is also mentioned several times, so to me there isn't a clear decision one way or the other. It'd be easy to make that change to semi-colons when I conflate county refs, I'm not 100% sure it's necessary. I'm flexible.

This is a good discussion to have before I make another pass through the data.