Many 27" bikes have brakes with long enough reach that they can be adjusted to work with 700c rims. Try it and see, if the brakes won't reach, there are calipers out there with more reach.
I doubt you'll notice any difference running 700c on the rear and 27" on the front, you're talking 4mm difference in axle height over the wheelbase of the bike, you can probably make it up with tire choice if you're that fussy about it.