Yes it's one of those things only a very few people wish for (for the time being).
I use BubbleUPnP from my phone as a control point and it allows me to search the media server. Don't know how extensive is its UPnP search (I can't test it), but JRiver being probably the best of its kind, should see that's it's not the last to implement such features. It's really a waste if all this work for such a powerful search system won't be extended to support DLNA requests also (well, if it's possible).
I know that maybe I could achieve this functionality from Gizmo, but would like to be using standards and be unbound from specific software cooperating with itself only.
PS: I was wrong that the search is currently working on filenames. Seems to be working on title (name) only. That's strange.. and maybe promising? I mean, if it works on a tag (which I suppose is more complex than just a filename), maybe it could be extended to other tags also?