My Pack has a similar rule about using the pre-cut slots, but they allow us to fill the slots and re-drill them as long as the holes are in the slot locations. So technically the nails are in the slots, they're just positioned within the slots at an angle based on re-drilling.
We also have a rule that the tip of the nail must be visible in the slot. So we accomplish this by drilling out a bit of the filler material from the slot after the the nails are installed.
I can't interpret your Pack rules to say whether this would be allowed or not, but if the rule is simply that the nails must be in the slots, my personal belief is that filled/redrilled holes within the slots would be ok. Of course check with your Pack first.
People fill the slots all the time, usually after the nails are installed to secure them in position. This is basically the same thing, just in reverse order, filling first, then installing nails.