Hi John!
Yes, you can use a 90mm guidescope.
I am doing this with an ETX90 with a STV guider connected. I guide a C14 with 2700mm focal length with a Caon 10D DSLR on it.
Works nice as long you have a bright guiding star within 0-3 degree off the object you want to expose.
To see results have a look at:
http://www.werbeagentur.org/oldwexi/...y_deepsky.html
I stopped the off axis guiding as i could very seldom find a guiding star.