Before you go about soldering on a strap ring, check out the following alternatives.
1. Are you sure it's simply the weight of the instrument that is causing the problem ? On my 1926 NWII soprano, the RH thumb ring was digging into my thumb painfully, because the saurface of the thumb ring was too hard and the edges too sharp for comfort. I solved this by lining the inside surface of the thumb ring with Sugru, which cushions the thumb very well, and eliminates the pain.
2. If you do decide to attach a strap ring, I'm sure you'll find that the best location for it is immediately above the RH thumb ring ! So, before you set about brazing a strap ring onto the instrument, try attaching your strap hook to the thumb ring itself. The thumb ring is large enough to accommodate both your thumb and the strap hook at the same time.
3. If you don't like the feel of the strap hook against your thumb, try the following work-around: Get a standard key ring and pass it through the thumb ring. You can attach your strap hook to the key ring instead of to the thumb ring. Then line the inside of the thumb ring with Sugru, placing the Sugru on top of the key ring, so that the key ring is fixed in one place, and your thumnb doesn't come in contact with it.
4. Investigate a Conn 4M Curvy. They're great fun, but for some reason when I play my Curvy (which dates from the same year as my NWII straight) what comes out is a softer, gentler sound than the one I get from from the straight soprano (which tends to be in-your-face, aggressive).
The point of all this is to modify the instrument in such a way that all modifications will be easily reversible.
Good luck - have fun !