WHOA--Don't drill anything. Even if you could never get the drain to work, you could always just unscrew the petcock from the core, or, just leave it and pull off the lower hose.

As for opening it, are you sure it is a 90 degree turn fitting? If I remember correctly, on my 99 430 the drain was a traditional automotive drain petcock that you needed to open with several complete turns. Even if it was open, if the system is so corroded that an open drain needs to be drilled out as the gunk is that hard, you need a new rad, as every tube in there is likely the same. However, I doubt that is the case, I just think you do not have the valve opened all the way.