For searching across all types of everything, I lean toward adding an "Insert object reference..." button to the header which pops up a search box (similar to the "Edit Blocking Tasks" box) instead of trying to trigger on {T... and such. I think this is a lot friendlier for new users, and we'll have a bit more room to let you filter/customize the query, and this operation is probably not so common that it needs to be triggered on a whim inline. For power users, we could maybe add a keystroke (say, Ctrl/Alt + @) or something so you didn't need to actually click the button.

I could see possibly doing an inline autocomplete for more types of stuctured objects (like wiki documents), but I'd be inclined to do the "Insert object reference..." workflow first and then see if it still felt necessary.

At least personally, I reference stuff I don't know the number something like a few times a day. My current workflow is to open a new window, use standard search tools, then copy/paste the reference back. This would be slightly nicer inline (and way nicer if I didn't know all the monogram/reference rules) but wouldn't feel too cumbersome to me if it was a button instead of an automatic triggered suggest.

We could also maybe do something like show a (non-interactive) hint when you type Txxx (i.e., put the full title under it in a little tooltip) so you can be sure you got the right object. I currently hover over the tag in the preview to double check if I'm not sure, which doesn't feel bad to me, but isn't particularly elegant. Or maybe we could stick the whole hovercard for the most-recently-typed object somewhere. This is getting a little far afield, but we're looking at making some changes to the comment box in connection with the two-column design.