Suppose you could use them in passive mode as I just had a look at the Tele set and there was no mention of any battery pack however there was a link to how to setup 9v or 18v battery power.

Have a look at Pablo's MMB4 to see how neat and tidy a job he did installing a battery pack in the back. Not so sure it wouldn't rattle about and maybe short things out if left unrestrained in the control cavity.