If I get some of that high-percentage silver solder and "tin" a piece of
brass with it, will I get a surface that can be polished and look like a
silver-plated bit of brass?

No. The silver content is too low.

I agree with Harold. I have tried several different brands of silver solder
and the color varies drastically from brass color to slight yellow tint.
The brass color solder contained cadinum and the lighest color was Harris
brand.

I have seen spots of very silver color on the part but I cannot determine
what causes them. They occur where there is extra flux and the solder
migrates a considerable distance from the joint. Must be some combination
of distance and heat.

chuck