Last night I went browsing bit-tech.net to see if they had any grill-cutting guides. The only guide I bookmarked is
here and it's in the modding section. The forums there are focused on modding and might be able to provide you with some assistance as there are guides and work logs and such.
Anyways, the link above takes you to a "meet your dremel" kind of guide where it also shows how to cut a circular hole with a cut-off wheel. Although I feel odd recommending another forum here,
bit-tech seems to be a great instructional place to see what others have done with their dremels.