TL;DR: A privilege should be added to be able to "vouch for" another user's usefulness to a particular question, giving them the ability to comment on that question.
Sometimes, especially in programming teams, someone who is very familiar with a piece of software may discover or be directed to a question that they are very qualified to solve. Of course, this could also apply to a game developer finding a question about their game on Arqade or a researcher finding a question about their area of expertise on Computer Science.SE. In some cases, this person may have trouble answering the question because they have insufficient reputation to ask for clarification.
I propose that we provide a mechanism for another user to vouch for this expert's expertise or ability, which would allow that user to post comments on the question. This should require significantly higher reputation than what is required to comment (I think the "Established user" privilege, at 1000, would be a good spot). Doing this would also not notify the user, to avoid the primary problem with "private message" and "ping" suggestions. Instead, the "vouching" user would have to notify the "expert" user out of band.