"tag:" should be able to return the amount of things owned with a tag
Let's say I wanted to multiply a building's yield by the amount of cool things™ you have.
If I say:
passive:multiply yield of coolBuilding by (1+(tag:coolThing))
the yield of my cool buildings™ should be multiplied by 1 plus the number of things I own with the tag "coolThing" - so if I had 3 things with the tag, cool building™ would be 4x as efficient