If you have the tools available you might try this.
Using 2 ‘third hand’ holders, solder stop and oxy/acetylene torch.
Practice on a jump ring with a tip that will allow completion of the
soldering in roughly 5 seconds.
Use the solder stop to fill in the gap between the jump ring and the
neck ring and while the stop is still wet, wedge tooth pick tips into
the gap to hold the parts apart. Use one third hand to hold the neck
ring, the other to hold the jump ring, or for the jump ring you could
use self closing tweezers. Wrap wet newspaper around the neck ring
and hold with a paper clip or binding wire. The real key to the
process is the solder stop, and the torch, and then mostly the skill
with the torch. Not knowing the sizes you are dealing with it is a
little hard to give specifics. Aim the heat away from the neck ring
That is how I would approach it.