Unfortunately, modifying or deleting files in a snapshot violate the whole point of the snapshot which is to have a unchangeable point-in-time copy of the volume.
But thinking a little outside the box, you could turn on nosnapdir, then instead of presenting snapshots for restores, you could spin off FlexClones which are read/write and therefore could be modified. It may be possible to write a script that spins off a clone, makes the modifications you want, then shares out the clone read-only. Restore would have to be done from the cloned volume instead of .snapshot, but the presented clone(s) could be based on the same snapshots (or a subset of them).
That may be reaching a bit and is, IMHO, a bit of an ugly kludge, but it might solve their specific problem.