I am experiencing the EXACT same scenario.
It's funny.. you are seeing the same thing I am currently having a problem with TODAY.
- I have a Pioneer deck
- I have the HF850 (bluetooth)
- I am replacing a wired handsfree kit (Motorola also) which the mute function worked flawlessly (just yesterday).
In my research all I have found is your post (with no reply) and I have learned that (just like the wired handsfree kit) is not a user selectable option on the HF850. IT IS JUST SUPPOSED TO WORK.
So... is my HF850 defective.. or has Motorola changed the signal type and I need to put a relay or converter kit on the Mute/Entertainment wire?
Also, robear, since this issue you were having is about 7 months ago, have you came up with a solution?