I think You are right. The question should be public in any case, not tooo private, but I still think it would be great if You could refer to a special user, so that he gets informed about the request. It would be a little bit like an expert someone wants to talk to.
A wall would be cleraly enough, it is just that the desired user gets informed. For example on Your , by the way very nice, site, if someone seems to be an expert for a very special thing, but was not online since weeks, how to ask him something?
If You could refer to him, he would be informed by mail and knows that his knowledge is needed urgently.
In my case I wanted to ask the user named R how he put in the questionbox the (onblur ) watermark input text. He and me had some communication about the questionbox on front before, and now I would need a small info by him. As he took parts of my code he just needs to send me two lines and it fits.
I could enter his site and ask, but as he has a targeted usergroup, I dont want to fill his site with non matching questions.
But however, a wall would do it.