It is awful hard to build significant muscle during the XC season. As a former XC runner and current coach, however, I can attest to the fact that it is possible to MAINTAIN muscle mass despite heavy running loads.
As you can no doubt guess from my stats, keeping weight on has always been a challenge, but I have had good success with really focusing on adequate rest and nutrition during the season (both for myself and my athletes). You definitely won't get big logging miles, but you can limit the damage and improve your running performance by attending to your caloric and nutritional needs. I highly recommend complex carbs 1-2 hours before workout with a carb/protein meal/shake immediately following. This, paired with solid nutrition throughout the day will do wonders.
If you want some motivation to sustain you through the season, think of it this way--when you hit the gym hard after the season ends you should really blow up (I usually do). I've noticed an effect similar to that experienced by depleted bodybuilders following the conclusion of the XC season and the resumption of heavy lifting.
Good luck!