Holes can also be used to relieve stress. In fact, it is common
practice to drill holes at the ends of stress cracks as a way of
stopping the growth of the crack. I believe the B-47 bomber's
spar was found to be too stiff, and it was only by drilling holes
in it that it became fit for service because the wing became more
flexible.
Back to your question: I think the important thing to know is
how big the holes are. Small holes of 1/4" or less likely are
not a problem, unless they are in a series around the
circumference of the mast. Welding can often weaken aluminum
because it creates localized heat stresses. In order to relieve
these stresses the entire mast would have to be annealed and
retempered. It just isn't worth it.
Bill Longyard