Ok, well if the tartrazine washes out, it indeed could be a good light restrainer. It also might be suitable for imbibition itself, which would be nice because it is an easy dye to get a hold of. Afterall, you're ingesting it everytime you drink a mountain dew!
Again though, I don't want anyone to think that they need to worry about light-restraining dyes at the moment. Last night I exposed & etched 2 new plain gelatin matrices and did a quick and dirty transfer. Preliminary results are very promising and I will post an example soon.
As for mordants, I know that thorium is not the only option, unless you're specifically referring to the use of Kodak cyan, which yes will probably shift without it. Not a big deal though, because the whole point here is to find a new set of dyes that can be worked into a scheme for hobbyists.
Mordants are another thing for 'down the road', but I know for a fact that fixed-out photo papers will work, and indeed, plain gelatin would even work. The key issue is the "dye absorption index". If the receiver absorbs dye very easily, diffusion of the image will occur and you'll lose sharpness. If the receiver has low dye absorption, then contrast will suffer. Technicolor tested the absorption indices of their gelatin blanks (film strips) and categorized them into different classes so that depending on their separation negatives, whether they were too hard or too flat, they could match it up with a complementary blank to produce consistent, balanced prints