For the military guys, if you get activated - at least at Air Japan - they will let you come back after your assignment. We had a new guy who got activated to Iraq after his training and was allowed to come back after his tour. Also, if you are with CREW or HACS, since they are both US companies they would have to let you come back if you get activated according to the US laws I presume. I'm not an expert on this subject, but I know for a fact Air Japan will let you comeback if you are activated. Also, the scheduling is flexible enough here where you can request days-off around reserve duty dates.
Hopes this helps a little.