I can't speak for other states, but in Florida's Ocala National Forest the trails, at least the ones you can drive on, are marked with rectangular brown highway shields, so ref seems appropriate. But you probably shouldn't leave out all prefixes - for example 43 is a tertiary road, and with simply ref=43 it wouldn't be clear that it's a forest road: http://www.openstreetmap.org/?lat=29.41438&lon=-81.68936&zoom=15&layers=M I'd find out what abbreviation is most common (NFS? NFD? FS? FR?) and use it as a prefix.
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