Ive never had, nor seen a hydrolocked Valkyrie...however I think you are assuming that fuel entering the vacuum line to #6 is the primary cause. I dont think that is the case. My understanding is fuel flows into the cylinder from whichever carburator has a stuck needle valve and fuel enters the carb from the normal fuel supply , not a vacuum line.
I agree.
For fuel to get to the vacuum line connecting the petcock to the #6 carb, the manual shut off either has to have been left open or the ball & detent system has to have failed, the diaphragm that seals the flow from the outlet to the fuel line has to have failed (or the spring that holds it shut needs to have slipped), the fuel line has to have filled to the point of backup, the weep/pressure relief hole has to be clogged, and there has to be a hole in the vacuum diaphragm.
Not an impossible combination, but not a probable one either.
Leaving the petcock in an "on" position, with a coincidental failure of the spring that holds the sealing diaphragm closed is the more likely path to hydrolock. (To the best of my knowledge, no one has ever discussed a hole in the sealing diaphram.) And, as Chris points out, some failure of the floats and/or fuel filling the carb rail vent lines must happen as well.
Two separate polls of this forum done a year or so apart both indicated that over 86% of the respondents had never had a problem with fuel in their cylinders. And, such polls tend to be biased toward negative responses from those who have had an issue, so I'm inclined to believe that actual incident rate is much lower than the 14% indicated by the polls.
Petcock failure resulting in hydrolock is much discussed and feared, which leads to the impression that it is a very frequent occurrence. It's not.
The more likely way the petcock will let you down is wear on the vacuum diagphragm, which eventually results in restriction in fuel flow to the point that the bike quits running. If there is a compelling reason to go to a manual petcock/Dan-Marc solenoid shutoff setup, this is it.