I don't think it would work. I don't have a bayonet and have never used one, but from what I understand, the ring in the crossguard of the bayonet slips over and into the recessed ring of the birdcage flash suppressor while the lug slips and locks the bayonet into place? If you mounted the bayonet such that it was pointing down there would be no ring for the bayonet to slip over and brace it against. The bayonet would simply fall off right? Maybe if it was a folding bayonet like some rifles have you could do it.
So... not only would it not work, but your bayonet lug would have to shift angles by 90 degrees and point down. How are you going to manage that? You'd need a special mount.
Then there is the whole safety issue of having a knife drawn like that. What happens if you dive into the ground? You could cut yourself or worse, stab yourself. The muzzle of the weapon is always going to be pointing away from the shooter so it's not terribly dangerous to put one pointing in that direction, but having one point down seem slike you're asking for trouble.
Why would you want to do it anyway? Just use a regular VFG.
Or is this all just a joke?