they hate clipping masks in Production, at least at DC, but nobody's found a better way to let art pop in front of lettering.
here's a little tip.
if you have to use a clipping mask, make a separate layer on your template. i call it "spare mask". i can hide the balloon layer, so that i can see what i'm doing, as i draw the clipping mask.
when it's done, copy it and paste it onto the balloon layer.
then hide the "spare mask" layer, but leave the original, and now invisible, mask in that layer.
why's this helpful? because once a balloon's been masked, it's awfully tough to manipulate. DC and Marvel and especially Vertigo constantly change their minds about stuff once the book's been lettered, and you have to guard yourself against their revisions. if a block of copy is rewritten, a new balloon may have to be drawn, or the old one manipulated.
so having a backup copy of a clipping mask, which may have taken half an hour or more to draw, is good protection. you can delete the old mask, make whatever changes you have to, and then copy the backup mask onto the balloon layer.
saves much aggravation. by the way, one of the great advantages of hand lettering, at least until 2003, was that if changes were made after the book was lettered, they were either done in-house, or if the letterer had to do them, the companies considered it your doing them a big favor, for which you'd often as not get paid.
some writers, particularly writers of creator-owned material, submit what appear to be second drafts. this took some getting used to…