How about coating the areas outside the part to be soldered with
ochre, rouge, whitening or some such which should prevent the gold
from flowing. You could probably make a 'mask' of a piece of brass
the same size and shape as the gold part and solder a wire to it
like a big earring and tell your folks to put it on the silver and
paint round it with water-based 'Liquid paper' or some such. Let it
dry before heating it.
Ian W. Wright