> What are the allergic affects of the tree, its raw sap, the liquid lacquer, and maybe for hypersenstive/reactive urushiol allergies, the finished lacquer?

Essentially the same as for any other urushiol.

I'm highly sensitive and had to ask my partner not to get into kintsugi with the traditional lacquers because even the tiniest spot of urushiol and I will be considering a trip to the burn unit.

I've gotten a very mild reaction from ~century old lacquerware but I wouldn't expect that to be common, once it's fully cured. And just because it's mild doesn't mean it's any less itchy, trust me.

see https://news.ycombinator.com/item?id=46268130