You can accomplish this with the adv_int.mo module which can ramp the interval time, but it would be a little complicated to setup. For example, for 300s subs you could do something like this:
keyframe @ time 1 with interval time=300s,
keyframe @ time 4 with interval time=300s,
keyframe @ time 5 with interval time=600s,
keyframe @ time 6 with interval time=300s,
keyframe @ time 10 with interval time=300s,
keyframe @ time 11 with interval time=600s,
etc.
This would give you 300s break every 5 frames
Perhaps I need a way to repeat a keyframe sequence that would make this a little easier. I think I'll add that.
I'm also planning to add a way to ramp the bulb timer with this module, but that's going to require some changes to the core.