Mizzie - if you can wait until October, Raúl will be appearing in "Speed-The-Plow" on Broadway and you can send your request directly to the theatre (the name of which hasn't been announced yet). He's always been very good about answering mail that way.
I haven't used International Response coupons in a long time, but as I recall they had to be taken to the post office and exchanged for actual postage/stamps. If that's still the same procedure, that won't work for your purposes. I'd check the Canada Post website and see what advice they have, or just go to your nearest post office and ask the postmaster there.
However, I think the best thing to do would be to put together the package, weigh it, and then go to the USPS website and determine how much postage from New York to wherever you are in Canada is, and see if you can buy US postage stamps in that amount (surely there must be SOME place online where you can do that!). And then enclose those stamps with your SASE and material you want to be signed.
Sarcasm is an allergic reaction to stupid people.