You seem to be using the term “virtual ground” to mean something other than what I understand it to mean, and I’m not at all sure I understand what you mean. Maybe if you draw us a picture? A schematic or at least some sort of diagram to make it clearer what you’re doing and where your concerns are.
But something like the MFOS wall wart supply is quite non-dangerous, if by “dangerous” you mean to your physical safety. (Your circuits’ physical safety is less easily assured.) The dangerous voltages are all confined to the wall wart. What’s on the wall wart’s 2-conductor output cable is 12 VAC, with neither conductor connected to earth. In the MFOS supply one of these conductors connects to that circuit’s ground, and hence to the ground of each module. If you have jacks mounted to an uninsulated metal case (or faceplate), and the jack sleeves are connected to the modules’ ground, then the case will also be connected to ground. If the case/faceplate is insulated (e.g. if the metal’s painted) it could be floating and you should connect it to ground. (Scrape a patch of paint off and solder a wire from there to ground on the MFOS circuit board. Oh, did you say “aluminum”? Forget I said “solder”, use a nut and bolt or something to make physical contact to bare metal.)
The cases of the pots (if they’re metal) should contact ground too. If the case is uninsulated metal and the pots are mounted to it, they will be grounded. If the case is insulated or non metallic, you may need to solder a ground wire to the case of each pot.
To reiterate, all this is not to do with your physical safety — that’s not in danger. It’s to do with making sure the electronics work.
It probably will not be necessary to connect the synth’s ground to wall voltage’s earth although it should be okay if it is (do it safely if you do it, and do it only if you’re sure you know what you’re doing. That has to do with your physical safety.)
You can use the continuity setting of your multimeter to verify contact between the case, ground for each module, ground for the MFOS supply, jack sleeves, and pot cases.