I'm interested in the cyclocross thing, but can't get a cross bike just yet. I'll be doing exactly what your thinking and others have done. I think the big thing is to have fun, and train (which will lead to more fun - you know not spitting up your lungs during the race
).
I'll probably just get some "road-like" tires for the bike. The gearing is actually already road-like, since I never got around to changing the chainrings out, and the bike is quite light so portage shouldn't be too big an issue (hardtail). Good luck to you!