[Tagging] Mechanism for linking to a traffic_sign object in the source tag?

[Nathan Edgars II]

I'm wondering if there's an accepted way of specifying that the sign 
being used as a source has been mapped. For example:
maxspeed=45 mph source:maxspeed=sign (node 43920231)
where node 43920231 is traffic_sign=maxspeed maxspeed=45 mph