You can surely get a mid length rear derailler and a much larger cassette. It would be helpful to know if you're on SRAM or Shimano.
That said, you can't be getting much use out of that 48 up front for cross racing. Even the pros are only using a 44-46. Why not drop that down? I use a 42 and am experimenting with a 40 (on the same cassette you use but single set up).
You could also take that 36 to a 34 but it won't have a lot of impact. 3 in the front is like 1 in the rear, roughly.
You're right, the 48 doesn't get a lot of use in races but it does get a lot of use on the road through out the rest of the year.
I'm on Shimano 10 Sp.
How about 36 - 46 running 11 - 30 on the back. You can pretty well run that all year round. If you run a mid length derailleur try to find one with a clutch to stop any chain slap.