I know I can search for all of a user's comments with “_ user:username”, but is it possible to search for all comments made *on* someone's works?

...Hello?

Waaaaoooo...nobody came, just me...and bye.

Other than opening each of their pieces (which should, in theory be tagged) there isn't a search for comments on an artist

Wow, that's a glaring missing feature. What makes matters worse is that the “Trac” link that they put up in Help doesn't even work. This site is a ruin.