Great idea! If Omnigraffle could export an .fxg or .ai file you could probably get the artwork into Catalyst through Illustrator. What formats does OmniGraffle export to?
Suggest you post the idea of Omnigraffle integration to the Flash Catalyst Wishlist/Ideas.
You can try exporting as PDF from OmniGraffle (you'll need to make a document with all the objects that you want from your stencil) and then opening the PDF in Illustrator and copy/pasting all of the elements from Illustrator to Catalyst.
FYI: PDFs usually open in Illustrator with a global clipping mask that you will need to remove before you can really get into the objects
Thank you both very much for the help, I really appreciate it! I will try to mess around with it and see what I can get to work. If I can find anything that works especially well I will report back in; I really think these stencils could be helpful with rapid concepting, prototyping and testing of interactions. Cheers everyone.