I would also like to see OHV as a standardized tag. Here in Colorado, many of our multi-use trails are open to OHV. I would envision something like:
highway=path
bicycle=designated
ohv=designated
horse=designated

See here for the wiki example:
http://wiki.openstreetmap.org/wiki/Tag:highway%3Dpath

What I don't understand is why the US would use:
bicycle=designated
and the UK would use:
bicycle=yes

Shouldn't there be one and only one standard?