I've put the mic that came with my HU on the drivers side of the cluster a little below the level of the seat. You can't see it and admittedly I don't use it, but as it is hidden the system looks just as professional as it was before.
Bear in mind that if you parallel your mic between two audio inputs then you are loading it more than either system expects and you may lose signal strength in both systems possibly meaning that you may have to shout to get it to work with both the HU and B&M. You might not, your scope will show you the loading effect.
I suspect that the threads that say it can't be done are looking to somehow extract the mic signal via the CAN bus. That presupposes that the B&M system actually puts out an audio stream on the bus and that the HU has the software to detect and use it.
So much assumption is made regarding the CAN bus. It's not appreciated that the CAN bus only passes data - it doesn't actually 'do' anything.