Yes I understand what you mean - sorry about the confusion.
I only use the mirror function for DNS - that is to say, my ns2 is a mirror of ns1 and those two are not used for anything else.
I would use rsync to back up everything else.
Have you tried doing something like this:
On the slave as root:
cp /etc/cron.d/ispc_web26 /etc/cron.d/ispc_web26.ORIG
## put something here... save and close file
chmod -x /etc/cron.d/ispc_web26
The mirror function on the master may not be able to overwrite the file then.
But maybe there would be errors.