This idea has potential.The last few USC seasons had a podium time bonus system; I don't remember what the exact rules were, but it made perfect sense back then. If we were to adopt something in that vein, though, I'd recommend revising the points system of ZakStunts as a whole. I'll illustrate why I think so.
Before the recent LTB rule change at least, I believe LTB was overpowered: you can gain +2 advantage through LTB in a race, but, given the linear scoring system, if you end it in second place the winner can only recover one point, and so one can build an unassailable advantage through LTB alone. The changes have lessened that problem somewhat, as they make it a bit harder to get LTB (for instance, under the previous rules our LTB score over Z243/4 would have been 4-2 rather than 2-1). Introducing bonuses for non-leading positions would likely affect the relative importance, though it is hard to tell in which direction, and thus we'd better consider the points system globally.
(On a tangential note, since we are reviewing various aspects of the rulebook before the next season, maybe we should consider changing the linear points system for something else...)