I just called ICBC and it's even more awesome than I thought, the guy was super helpful:

I can go to any autoplan broker, tell them I need to make a pre-payment and can pay any random amount of money with my credit card, and then ICBC will just deduct it from the balance when my payment is due. I don't even have to "time" it with my renewal date, I can do it anytime I want! That's sweet. So I can just pay month-to-month until I have a big minimum spend coming up, pay a large pre-payment to fullfill the spend and then don't have to worry about ICBC for a while.

Sweeeeeeeet.

Now if only Mazda was as cooperative... Maybe if I go to my dealer I can make a pre-payment too, but I'm not sure :lol