I don't think it should be tied to the outlining mechanism since there might be other cases where you want to display adornments around other non-outlined lines. Although I do agree, in general there will be a lot of overlap there.
I agree that tagging a range via a new special line-based adornment tag would be the way to go. And we would need to support the potential of multiple adornments on the same line.
Say that you have a tag range that covers a method, which is over 3 lines. If your tag says the adornment should be above the line, that's fine and it would reserve space above the top line. But if the tag says the adornment should be below the line, would it be below the first line or below the last line? I'm thinking that it should be below the bottom line of the tagged range in that case. Do you agree?