Bob,
Silver electroplating is the easiest way to do what you are looking for. File the shape, polish, silver plate (heavy), and then polish off the silver where you don't want it. The more correct way to do this would be to prep the brass and then mask the areas you don't want to plate (by painting with asphaltum varnish - just asphalt and paint thinner). Then plate as usual and wash off the mask with acetone or thinner or whatever solvent is handy.
Another, more period correct way, is to make up a small amount of an amalgam of silver and mercury (same stuff they jam in your teeth) but use a little more mercury than the dentist does. Rub the brass with it and the mercury will join the silver to the brass. (It will look like it is smearing all over the surface). Take it outside some place with plenty of ventilation and put it on a hot plate and let the part get to a dull red. Don't breath any of the fumes that come off. When it gets red, turn off the hot plate and let it all cool down. The brass part will be coated in a fairly thick coat of silver. Polish it off anywhere you don't want it and burnish it where you want to keep it.
Now before you get a bunch of frantic responses about mercury poisioning (ad infinitum, ad nauseum), if you don't suck up a huge lung full of mercury vapor, it's not a problem. I have been using amalgamated gold and silver for 50 years. Most of us have a mouth full of silver amalgam. If your afraid of mercury, don't use the fire gilding method just described.
The silver solder method you described is possible, but not very controlable. You might have better luck using a low fusing silver solder and attaching very thin silver plates where you want them.
