Thank you for your reply!
First i want to clarify...it is a Squier Tele Custom i am trying to modify.
Yes, the jack will be the strap button (i hope!) and yes, i meant the control cavity..also, the typical output jack hole is already present, but i'm going to be using it for something else.
I was thinking about drilling straight through to the pickup cavity too, but the string ferrule holes and bridge mounting screws are in the way, just like you said.
That made me start thinking about taking a sharp right from the endpin hole to the control cavity, but at this point, i'm not feeling very confident about being able to drill either one of these holes properly.
I have very little experience with this kind of stuff (i'm just getting started) so i'm having trouble imagining what i might use to keep the drill straight for both the endpin jack, and the hole that will be going from the cavity to the endpin hole (starting from outside the typical output jack hole).
Here is a picture (from the Stewart Macdonald site) of the endpin jack i have.