I've not drilled them, but from what I've seen of the mud up here (mostly clay) I can't believe that holes would help much. Maybe you could reduce the diameter of the whole plate so that centrifugal force could clear some of it out. Or just run commando, without any plate to speak of. Might be worth a try anyhow...
'75 EB 302,3spd,35s,5-1/2" lift,ps,front disc,DUI ign.,Edelbrock intake/carb
'72 in parts waiting for a miracle.