Well, it's a close approximation to real life. We are essentially measuring the rads absorbed when in these 'pockets' of radiation, as you say, and tracking it over a certain period of time. Too much radiation over, say, a 12 hour period, and you suffer some ill effects. That might also be a way to do it. Have the server track how many rads something has absorbed over X hours. It would only take 1 more field per *relevant* object. Then after a certain number of ticks it updates with radiation effects. Also, if we end up using curves a la Planetary Humiliation, this makes keeping track of rads/time easier. ((t=12341, enterered rad4 space), (t=12387, entered rad3 space), (t=12400, entered rad0 space)) would clearly mean that it would have been exposed to 223 rads recently.
Also, this is a 'close approximation' if you consider the only difference is we're tracking an interval rather than the entire life of the object.