Hi Daniel,
I think having a mapillary tag that shows how the POI/object looks like from the street is perfectly fine, and is indeed how the mapillary tag is used in most cases.

"streetsign:mapillary" is a bit problematic syntax-wise: There is no "streetsign" key it could refer to. We do have "image:streetsign" which is only used on highways to show their name signs - which is perfectly fine because "image" without subkey would imply an image of the road itself. 

I think it stretches the definition of "streetsign" in OSM a bit to include "signs of a POI that are visible from the street". 
