I found that if you use the rubber feet on the bottom of the speaker to tilt it slightly away from the wall there's enough room under the speaker for the wire to come out of the wall almost anywhere behind the speaker and easily bend around to connect to the posts. (I use 14 gauge wire)
The trick I had to deal with was that I am using wall boxes (the retro boxes that just snap into the drywall). The T-brackets cannot be mounted over the wall plate in any way or it is not flat enough to successfully mount the speaker, so I ended up putting the corner of the T-bracket (one of the corners under the 'top' of the T) right next to the top of the wall box insert. i.e. the top of the T extended over the top of the box and the middle of the T runs down parallel to it. (This does successfully hide the plate and the wire by the way)
I'm not sure about just mounting the horizontal part. If you were in a horizontal stud and all screws went into a stud then it'd probably be fine, if not I'd prefer to have something screwed in going down the wall too, but even then it's probably not an absolute must. With some good drywall screws you could probably get away with it either way. Not much of a definitive answer I know... without having tried it I'm not sure either.
