I live in Canada, was on a 18month wait list if I was lucky. Used my US address, paid the premium and now I am the proud owner of my R8. One major problem where do you find 100 octane gas in a smaller town? What does everyone think of Ocatne booster?
95 RON is equivalent to 90-91 MON. So, basically, a 91 octane gas in the U.S. (which uses the MON method) will perform fine in the R8. And a 93 octane, which is readily available in most of the U.S., is equivalent to at least a 97 octane gas in Europe.
A 91 RON is the equivalent of the standard grade gas in the U.S. (87 octane) and, as you say, should be avoided except in an emergency.
Although the octane rating of premium does vary slightly by region in the U.S., it's almost never lower than 91. So, basically, you can pull up to any premium pump anywhere in the U.S. and gas up the R8 with no risk to the car.