My Morgan has an after market LR Baggs pickup fitted. It has one of those end-pins that includes the jack and the battery switch.
I don't know why strap manufacturers don't offer a straps with holes pre-cut to handle this situation, but they don't. And cutting it yourself is an inexact science. My strap fits over the end-pin, but can slip.
So today I made myself a little clip that fits over the strap after it's been fitted. Almost like a lock - should stop it slipping.
I cut a square out of an expired credit card and drilled a hole thru it with a 12mm bit. Then I cut a slot from the outside into the hole to give a key hole.
This then slides over the strap but still engages under the edges of the end pin and thus reduces slippage.
Attached (hopefully) are two images. One shows the clip by itself so that you can see how it is made - and it's real simple.
The second shows it in use, locking the strap in place. This kind of plastic works well, but will probably get "plastic fatigue" after a while. So I'm going to root around for old medical aid cards etc and make a whole bunch of these.
(err.... ja..... well.... I know I did this before, but I can't figure out how to attach the images, so I shoved them up to picasa and added links to them).