I definitely recall the idea, as well as the picture posted. And I found the idea interesting and was planning on following how it played out (should it be pursued, and come to full fruition).
Honestly, while I’m always interested in new ideas, methodologies, and advancements to user conveniences… I can’t personally see any benefit that additional stirring could provide, when the cavitation process of the ultrasonic waves is already doing the work at a “molecular level” (slamming the PG/VG/flavoring/nic into each other at the speed of sound literally).
Additionally, I would posit that the stirring device (assumed mounted in a center position) would only interrupt/conflict with, the pattern being generated by the US waves. As the transducer in most US units is mounted in the center, under the bottom of the pan. IE: mine is is smaller, square unit (holds about 9 30ml bottles, which I submerse to the top of the neck, without touching the cap) and the wave pattern that is generated travels from dead center to within about a half inch of the walls (when empty). When loaded with bottles, that pattern is somewhat affected (naturally) but I tend to arrange things in a geometric pattern, and they’re not usually far off from that when I return (anywhere from 1 hour later - fruits-, to two or three hours later - custards/creams-). The only times I typically see things being unlike I left them, is if I overload it with 15ml bottles. (TLDR: the waves are minimally affected, unless I “violate” the capabilities of the machine)
At any rate, the waves pass through glass like it wasn’t there. Not the case with plastics (as we know), which is why an ultrasonic isn’t recommended to folks who don’t use glass, as the process is greatly diminished. Obviously it’s not going to do so (waves pass through) a metal rod that’s rotating. That’s a point of disruption.
In my mind, unless the stirring device was timed (in mathematical precision) with the generated waves, I would be surprised to learn that the process would be helped. As I would expect the opposite to occur.
Perhaps the stirring offsets the loss of some waves effects?! Hard to say!
But it’s all a fart in the wind without testing to back it up! So I’ll be happy to watch your progress, and read your thoughts and observations, as things continue to unfold!