If you want it to look nice/new/fresh/exciting, you will not be able to get it this way without removing the wheel. There is just no way of completely cleaning/coating the caliper without removing the wheel. Your best bet to get them cleaned up is to:
- Remove the wheel
- Pull the caliper from it's mount
- Remove the pads (keeping them in positional order for proper replacement)
- Take a wire brush to it to knock as much of the rust off as possible
- Blow the caliper clean of debris with some high pressure air
- Mask any areas of the caliper/car/rotor/whatever, so as to avoid overspray on these areas
- Spray the caliper with hi-temp primer
- Allow to dry to tacky (probably about 10 minutes)
- Spray the caliper with your color choice of hi-temp paint
- Wait until completely dry, then re-assemble
What it comes down to is you can either have a good looking caliper, or a half-arsed job - it's up to you. Without taking the wheel off of the vehicle, you WILL have the latter.
If you are actually talking about the rotors and not the calipers (or both), your best bet is to replace the old rotors with ones which are zinc coated. This will maintain a nice appearance in the non-wear areas for a long time to come. This still entails removing the wheel, though.