Whimsically speaking: why don't you mount the Bronica on a seismograph and see if the the camera vibrates or is merely transmitting the force to the (undampened as noted before) water which is only showing its own long term instability.
Anyone got a spare seismograph? LOL :whistling:
First of all, the vibration is neither restrained, as it would be with a good tripod, nor damped as it would be held in the hands. Like the way a cell phone set to vibrate will jump around on a table but not in your hand.
An empty glass struck with a hard object will certainly vibrate, but you can't see it. Put water in it and strike it exactly the same way with the same force, and you will clearly see a strong disturbance in the water.
Your water test is useful as a comparative measurement but not much as an actual measurement.
I dunno but I've had a ETRS for a few decades as my main camera and NEVER seen any evidence of mirror slap or vibration even when doing macro with lens wide open. I have heard this fairy tale before about the Bronica but on challenge never had anyone who claimed it is an issue actually provide proof and I've loaned mine to them to test. I know a number of professional photographers who used the SQ and ETRS as their main tools of trade and and while some lusted over the likes of a Hassey (even I do from time to time), none ever though the camera was a limiting factor in their work.
I also had an Exacta system with the same mirror slap and even with 35mm, there was no evidence of a problem b/c of the design.
My SQ-A brutally slaps compared to P6 or M645 1000s or RB67. That matters only for 1/30 and below for 80mm and longer focal lengths. when mirror is locked up I really haven't fell any sort of vibration with lens shutter.
I didn't quite understand the OP but if he seems to have the same problem with all lenses he owns then perhaps it's time to switch to Hassleblad.