Corwin,
Sorry, I realised the omission and edited my post after you read it. I just checked the Amana website, and there's even a 7/32 bit that would fit the 6mm just perfectly.
Point being: you''ll want a close enough fit - "same radius" fit wouldn't even be preferrable.
Where's the benefit in Festool making fractional cutters ? They sell Domino's for every cuttersize. And if you want to make your own dominostock: that's exactly where you are: on your own.
Strength- and sizewise, they've got it covered for the size of the machine. So no reason to add extra sizes "in between" only to accomodate users that want to make their own stock.
and from a technical point of view, there's little reason for cutter manufacturers to make fractional mortise cutters, as it's easy to find bullnose bits that will do the job just fine for the metric mortises.
For clarity: I don't own a Domino, but i have done more than a few rounded loose tenons - through-and-through visible joints as well.
Regards,
Job