Mike: I'd just be concerned that the zip ties might shift over time and with temperature changes as the motor heats up/cools down. But we probably won't really know the answer until you try it! Andy.
You might want to consider the APPLE IPhone app called "iSeisomometer" instead..... Not only will it sense one three planes but it will allow you to include the "g" force factor which will amplify the waveforms making it easier to work with. Typically you can ignore the vertical, i.e., "z" axis data since it cannot be resolved by motor balancing.....but it tends to minimize when "x" and "y" plane vibrations are minimized. The problem with a microphone is bandwidth, i.e., cannot sense ultrasonic and especially subsonic frequencies.....and you need to keep an eye on subsonic....., i.e., 0 to 20 hz.
I decided to switch them over to a small piece of led tape after my flight to Brazil this week. After the temperature change from being in Toronto, flying to Manaus and being in the 35+ degree heat here, the zip ties were already loose on the motors. The tape on the motors has made a big difference though. I also switched to an allied drones echo stabilizer at the same time, but the combination has made big improvements for me.
Andy: I had a similar experience as you and what I found was my actual FPV camera was the culprit. I had the one from Iftron. I replaced it with an identical one and it works fine.