Jem 3 way switch?

I haven't done it on a JEM, but it shouldn't be a very big deal to drop a three-way blade switch in there. (A LP-style 3-way switch would be more work.) You can wire it any way you want. It would actually make it a lot more like the JS wiring.

Or you could simply leave the 5-way switch there and change the wiring to give you the bridge/neck combinations you want, and skip the middle pup connections. That's just a wiring change. Pretty simple to do, once you figure out exactly what you want it to do.

I personally love the middle pickup on my 7vwh. It is quiet and it cleans up nicely. So what I would do is add a switch that makes it so whenever the bridge or neck pup is on, they are both on. Have one of them on my G&L and its sweet - I can get that middle position tele tone with it.