But here it's a guidepost solely for bicycles. I know the problem you describe, but that's not a solution. Data consumers can't do anything with it, and it's unsuitable e.g. for screenreaders.

Seems like it should be something like direction_northeast:bicylcle (but only if it's ambiguous). But I don't know if that combination is actually commonly used/supported yet. Found one similar thing for mtb: https://taginfo.openstreetmap.org/keys/direction_east%253Amtb