It's hard to know how you would be able to undertake much, if any CFS work through the fire season Robert.
My very vague understanding is that the towers are manned by contractors, who no doubt have to be up there every day over a certain fire danger level (moderate? high?), which equates to most of the fire season.
You'd have to ensure that any CFS jobs you attended wouldn't affect your ability to man the tower, and as all the good ones this year happened through the day (the ones I can remember anyway), you'd not get much hands on action.
Mind you, it's an important job and a single tower person can have a hell of a lot more influence on a fire than a single fire fighter.