I'm assuming that your intent is to build a Warmoth rather than mod a Gibson LP Jr.? If that's the case, just order that pickup routing on a flat top LP body and you can get the pickguard made custom from Warmoth or somewhere else.
It would get tricky to do that mod to a Gibson model as besides having to route for the pickup, the control cavity isn't designed to accomdate the 3-way switch and you have to expand it and make a custom cover.