I'm not (yet) expert in casting silver, but I've got the stuff to try with, so I might do it and report. I use borax as a flux for my bullet casting (or in mixture with boric acid).
I also use it in silver soldering and brazing. Flux for that process will definitely work well. If you can't find it at the hardware or welding store, McMaster-Carr has everything and is a good outfit to deal with.
http://www.mcmaster.com/#
They have so much stuff you just about have to use the search function. Their catalog is 4" thick - on bible paper.
A general problem with hot metals is surface area/volume ratio. Small amounts are harder than large amounts, because they have more surface area to oxidize compared to the total amount - thus the percentage losses are greater.
I have a huge box of worn out keyboards (you can guess why) and am planning to strip them and melt them, though, just to play with the idea. This was mentioned here elsewhere, but rather than chemically remove the silver with acid and making silver nitrate, I plan to just use a solvent and scraper and leave it as metallic dust mixed with a bit of plastic. As I heat it, the plastic will decompose and mostly leave carbon which is a reducing agent itself, and I shouldn't have to put borax on it till we get pretty hot - but that's what I'm going to find out.
For extreme cases, I have a Vulcan heat treatment oven, programmable, I can just fill with inert atmosphere (say, argon) to do this - it will go to about 1000C. In that case, no flux is required.
I can also make my own crucibles out of various things - quartz or pure carbon, for hi temp work. In the case of quartz, I can custom-make something I can again control the atmosphere in - for example, pull a vacuum on it, again eliminating oxidation losses. Or use a hydrogen fill to reduce any silver compounds back to pure silver at high heat.