I know that board member/expert kladue has done this. I had asked him about it just a short whil back. Listed below are the instructions he gave to me.
Yes you can silver solder with 56% silver solder and the stay silv flux, the propane burning "turbo torches will produce enough heat to get the job done. Solder used was Harris safety Silv 56, flux was Harris Stay Silv white flux. Supplies will cost around $30 for small container of solder and ~ $4-$5 for flux which will last quite a while. you can thin flux with water to make a thin slurry to coat area to be heated, should leave a white layer on metal. Spend a little time in joint fit up to try and make joint gap as small as practical for stronger joints with less solder. The solder and flux are available at the Airgas facility on McGilchrest street by the airport, an equal product by other supplier will work the same, just make sure solder is cadmium free.
Step 1: clean area where solder joint will be with scotch brite.
Step 2: Coat area where solder joint is on both sides of keg and fitting, make area coated at least 1" wider than joint area to limit heat effects on surrounding metal.
Step 3: Apply heat by fanning flame across joint area slowly to prevent overheating, flux will change from white to transparent as temperature rises, watch metal color and try not to go lighter than dark red.
Step 4: Apply end of solder to joint and fan flame across end of solder and solder should turn liquid and flow into joint, solder will also flow towards heat and could flow away from target area if you are not carefull. If metal turns black from too much heat, cool metal down and sandpaper bright again and start over from step 2.
Step 5: After joint is left to cool naturally, use hot water and stiff brush to remove flux residue fom both sides of solder joint.