I commend you for being aware of the volleyball energy your daughter is expending and wanting to keep her from burning out.
The 9 month club schedule is a bit crazy and I wish USA Volleyball would put some parameters on the season, as to allow for some down time for all involved (just like the NCAA does). But, for the time being, Volleyball for Junior High and High School age players can be a 12 month cycle.
A few thoughts:
1. She has passion and wants to be in the gym, so I would let her go. Kinda like the little kid who just wants to run and run on the playground, you just gotta let them go with it. Don't try to reign her in now.
2. Because young ones can't keep the throttle open full go all the time, she will start to fade a bit at some juncture down the road, and it is here that you need to let her slow down. I think parents sometimes get a little dyslexic with their VB players and try to slow them down when they should be running, and then try to make them run when they should be slowing down. If she expresses hesitation about going to some "volunteer" workout, or additional training session, then follow that hint and let her know it is OK to take this training session off to recharge.
3. As Junior High Volleyball seems to just be passes and giggles, it is not a bad thing this season is not intense. I am getting more and more emails from PSA's who are taking high school volleyball completely off, as to have a down time during year, away from Volleyball. As your daughter is getting touches and is having fun, but not in a crazy or stressed training/playing environment, then this can be her slower time.
4. I would stay away from any physical training as your daughter is only in 7th grade. One sure way to burn kids out is to put them into heavy cardio or physical training environments, early in their career and in conjunction with their volleyball training. The body and the mind need time to decompress, especially at such a young age.
5. Encourage her to pursue/enjoy non-volleyball activities; when something becomes a person's entire world, then things get out of balance; just look at college coaches who seem a bit crazy....too much volleyball. If she diversifies a bit with art, drama, tennis, swimming, mowing the lawn, etc., then when she is in the gym, it will be fresher.
Good luck this club season!