At that point, you aren't driving the LEDs by constant current. You would have to very closely match the power supply voltage to the total LED forward voltage, and pick a power resistor of the right size to limit current. It's not very efficient at that point, and a little cumbersome in setting things up. A constant current LED driver doesn't have to be that picky when it comes to choosing a power supply based on the array size. For the most part, if you are using an AC driver, all you need is to make sure that you have the LEDs within the limits of the driver. With what you are proposing, you don't get that kind of flexibility. You also lose any protection that the driver would offer.