For anyone who's curious, I ran a test program. I was able to make Mastercam insert the Z values for pecking by choosing multiple points at the same X/Y location, but different Z locations. Then I manually inserted a magical, top-secret, 'E' parameter (with value 0) into the G84 command and this orients the spindle to the same spot before each peck. I haven't cut any metal yet, but I'm confident enough to run the part with this.