I know nothing about Mastercam or its post processors. One suggestion though: could you live with the Tool number transformed into a line number? I'm thinking maybe you can figure out how to grab the tool number and prepend an 'N' onto it. This won't give you a multiple of 10 as you were thinking, but it may also be handy to search through a program based on what tool you were thinking of calling up.
Another fellow showed me a trick with a post processor in another program: turn line numbering on, and then forbid line numbering for lines beginning with all letters of the alphabet except 'T'. This works well enough for searching through a program by just looking for any instance of 'N'. This gives unique line numbers that are not in any predictable sequence except that they are increasing.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)