My honest opinion? Forget it. As I said, you can not make a separation of precious metals via the cupellation process.
Lead is generally added as litharge, which oxidizes the ore samples and then collects values with the resulting lead. That would be in the first process after crushing the ore. It would not likely be used when assaying extracted metals, but more so when extracting from ores. That may or may not be true in all cases. It's possible that the litharge that comes from the cupellation process (the lead that collected the values is converted back to litharge in cupellation) acts as a carrier for the base metal oxides and allows them to be absorbed by the cupel, along with the litharge. Please note that I said I paid to have any assays run by an assay lab, so I'm not up to speed on assays, although I have a basic understanding of how they work.
Silver is used for the inquartation process, so gold can be parted with nitric acid, removing traces of base metals and all the silver. The percentage of added silver is critical in that it's very desirable for the coronet to not break in to pieces or fine particles. It is hoped that all base metal and silver can be dissolved, leaving behind a barren honeycombed representative of the coronet, which would be gold and other precious metals.
Assuming you have to inquart your gold to get to the point of purification, the only thing lacking at that point is actually dissolving the gold in AR, then going through a proper precipitation. By that method, you can separate the platinum group metals from your gold. That may not sound like a problem, but if you get involved with dental gold, or even jewelry wastes, platinum and palladium are a fact of life, and can add appreciably to your stores of precious metals when handled properly. Further, you don't really want to leave them in gold, for each of them toughen gold considerably. That's why they're used in dental gold. Once alloyed with gold, the gold is not very desirable for use in jewelry.
In order to process platinum group metals, you almost have to get involved in silver refining----assuming your source is dental or jewelry. Both will contain silver, which becomes the carrier of the majority of the platinum group, like it or not. Even platinum will dissolve in nitric to some degree---when in the presence of silver. You recover values from the sludge from the silver parting cell. They're difficult to recover otherwise.
One thing to consider. If, by chance, you want to improve gold by absorbing oxides with flux, DO NOT USE SODA ASH. It is a reducer, and will put oxides right back in the alloy as clean metals. It's used in recovering values from waste material routinely. I posted on that subject tonight, a recipe for flux. Please follow this link to read my opinion if you're interested. viewtopic.php?p=1844#1844