If I'm understanding you correctly, I haven't seen a bracket that will attach to the TV mount.
Here's a "plaque" I built for my QS8 speaker, because in my 120 year-old home, I have horsehair plaster and the studs are not regular. I didn't want the mounting screws to start vibrating out so I was looking to distribute the weight of the speaker over a larger mounting area. It's what I had in mind regarding you building something to bridge between the studs...though yours would need to be wider I'm sure. In this case, I put a hole through it and a "channel" in the back to place the wire.
