I recently bought a strap and have this same problem. I actually I think I found that same thread earlier. I have a 314ce by the way, so it has the ES end pin.
Anyway, I had an idea about this, but haven't been able to try it yet. I'll try to explain it here.
1. Get a piece of fairly strong string. I fly sport kites, and will probably use sleeved bridle line, rated at about 80 pounds test. You'd want something thin enough to fit comfortably in the end pin slot, but strong enough to support the guitar.
2. Tie a loop in the string.
3. Get a small (1cm or so) wooden disk. A button might work. Drill a hole in the center. If using a button, the holes in the button will be too small, and drilling them out may leave you with a hole too big.
4. Slide the loop of string through the hole, so that the knot in the loop locks at the button. So, you end up with a loop of string with a button at the end. The knot in the string needs to be big enough that it can't slip through the hole in the disk/button.
5. Either loop the string twice around the end pin slot, or use a larkshead knot to attach the loop to the end pin. Larkshead knot:
http://www.youtube.com/watch?v=WyzUmetVJFA6. Slip the button through the hole in your guitar strap, same as you would normally do with the end pin.
So, you're essentially adding a second end pin (the disk/button) to the guitar. The idea may need to be tweaked to be sure it's secure enough, but it seems like it should work.
Also, both double-wrapping the loop around the end pin and using a larkshead knot will actually use the weight of the guitar to tighten the string around the end pin. So, the string won't come off. The weak point would be the knot pulling through the hole in the disk/button, or the disk/button breaking. So I would think these would be the things to tweak, if necessary. Start with a bulkier knot, or maybe a second, smaller disk/button on the outside of the main one.
What do you guys think? I've been thinking about posting this on the acoustic guitar forum, since there are more people, for more input.
And of course, there's a fair chance someone else has already done this.
Jason