Indeed, I just got a pair of Mackie in-ear monitor headphones that use MMCX connectors. You can swap out the headphone cord for a bluetooth adapter and mic, using that connector, for instance.

I don't have an answer to your question at the moment. However if I were in your shoes, with that particular pair of headphones, I'd stick with the 3.5mm adapter as designed.

Reasons:
- I find the MMCX connector to be very fiddly to use. I have trouble plugging them in and knowing if they've snapped in properly or if I need to still apply more force to get them in. I'm always afraid to break them.
- If I needed something with an inline mic, I'd just swap to that kind of headset. (On those Mackies I just got, I played with the BT+Mic adapter and the latency delay is just too much for me to make use of it.)
- 1/8" cords are easier and cheaper to find and source than MMCX cords. Especially if you've got new toothy kittens.