Thank you all. I needed all of those tricks. I did widen the slit a bit with a knife. Then the water/nickels/vice grip method worked.
And it had the added benefit of embossing Thomas Jefferson's portrait on the strap.

I positioned Monticello appropriately on the back of the strap, of course. Since most of the embossing is either over the hole or hidden when it's on the guitar, only I can see it, but Tom and I know he's there!
Thanks again for all your suggestions. This is a great site!
As for ggj's post, which came in while I was typing mine, all I can say is do some/all of the suggestions and it'll work, provided the actual hole in the strap is large enough. If not, seems you need to try punching the hole slightly larger. X-acto knife or a leather punch would do the trick. If it's too thick, I would think the Vice Grip of Death torture would make the leather yield to your will.