Ah, I vaguely remember having this conversation before… I think my big problem here is that the old version of search (with the checkbox to toggle searching in the current context versus globally) was able to handle this. In a sense, that simpler and less pretty version was more powerful, because you could easily toggle the context no matter what you were searching for. That’s one of the two reasons I liked the old version better, despite the new version looking nicer. (The second reason being that Enter now defaults to global search, but 90% of the time I want to do contextual search.)
It looks like it already works when searching just for an exact username, specifically when searching the current topic.
Doesn’t work on the category page though.
In general though, I think it would be good to support as much as possible when there is an exact match. It may be easy to add #contribute:feature as you suggested, but searching in the current topic is less straightforward. I just noticed that searching some text @seanblue in the current topic works, but searching @seanblue some text doesn’t. So I guess the current topic bit does partly work at least.
Anyway, everything I just wrote was kind of steam of consciousness. I haven’t really thought though the whole picture beyond “it would be nice if it always worked”. Besides separating out the contextual/global toggle again like the checkbox used to do, I don’t have any ideas for how to handle this simply in all cases.