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.