
You don't need to ask for permission, some judges will knock it, others may not. A dog of equal quality with a full mouth of teeth will probably beat it.
Lots of dogs in the ring with missing or damaged teeth. I don't think its something to get a vet letter about as there is no point at which you could appropriately produce it - before you go in? Then the judge would be looking for it and may not have noticed, in the ring? - not appropriate) Plus it would have your and dogs name on which is against the rules.
I would just enter some and see how you go :)