Huzzah! I seem to have figured it out on my own!
Used a hex editor (XVI32
is nice and lightweight) to open xampp-control.exe and change the following:
As you can see, I've just added a "2" to the end of "MySQL" in that area. As far as I can tell, this is the section of the code which tells XCP what process/service name to look for with regards to the Start/Stop MySQL button. Saved the file and changed the socket name from "MySQL" to "MySQL2" in my.ini (two instances near top of file) and php.ini (two instances - mysql and mysqli sections).
XCP works perfectly now - the MySQL Start/Stop button controls XAMPP's version of MySQL, and the version running on the PC is unaffected.
Hopefully this will help others!