These pictures lead me to believe that dental bands would be too thick for this. The RF rings are so much smaller than o-rings.
Nice pics! I would assume the only downfall of the dental bands mod (compared to standard type-S) is that it affects travel distance by an extra 0.5mm or something?
You could always make your own o rings.
http://geekhack.org/index.php?topic=40582
I can confirm that this mod works great (punching your own rings out of thin rubber), but it's not particularly fun to do. Plus, an 87U has like eighty thousand screws in it, all of which are fairly tight from the factory, so if you aren't careful you could easily strip a lot of them.
It doesn't affect the feel of the keys at all, and it reduces sound about as much as the from-the-factory silenced switches (I typed on a silenced-in-this-fashion and an unsilenced 87U side-by-side, and am typing this on purple-stemmed silenced topres).
Plus, (and I don't think this is an issue for the HHKB or the Leopold), the left control key of the 87U has an LED and is a different plunger design than the rest of the keys, so you can't just add a ring to it (there's sort of a slot that the plunger fits in to in order to fit the led in there). I use the left control a lot (emacs) and so I got around this by desoldering said LED and swapping the switch housing with the normal one from of the keys I rarely use (I believe pause/break).