I know zero about this topic, but I found this interesting...
Urine has often been used as a mordant to help prepare textiles, especially wool, for dyeing. In the Scottish Highlands and Hebrides, the process of "waulking" (fulling) woven wool is preceded by soaking in urine, preferably infantile.
Another great use for pee. That may make it "perhaps the easiest mordant to make at home".
Pee also makes a great modifier in natural dyeing - in that the ammonia in urine changes the PH of the dye bath to change the colour of the plant dye.
I haven't seen it used as a mordant. Modern mordants are usually minerals like copper, aluminium, iron, or chrome, that bind with the fibre and encourage the dye to bind with the mordant. But I haven't seen everything and I suspect urine has trace minerals in it, so it might work. I'll have to do more research and see what I can learn.
A mordant is a metal ion which attaches to the fiber, usually by being boiled together for a length of time. A dye which has no natural attraction to the fiber can then attach to the metal ion. Most but not all natural dyes are mordant dyes, which require the metal ion to be in the fiber in order for them to have any attraction to the fiber.
It also mentions that tanic acid also works as a mordant on some fibres.
here's another source that confirms urine can be used as a mordant
Traditional natural dye mordants with a capital Pee
Traditionally professional natural dyers used chromium and tin, along with the less toxic mordants — alum, copper and iron, as well as stale urine. Today chromium and tin are known to be dangerous and toxic to the dyer and the environment even in micro-grams and natural dyers do well to avoid them. Many beautiful, vibrant and permanent colours can be achieved with using only alum, with afterdips in rust water, to sadden the colour, or copper water (copper pipe left to sit in an ammonia solution.) Urine has a strong ammonia odour from the urea dioxide and nitrous oxide in it. It can be beneficial when dyeing with indigo or woad, especially. But dye with it outside and away from human dwellings. Urine can also be used as a mordant for other natural dyes. My favourite natural dye book, A Dyer’s Manual by Jill Goodwin, (brought back in to print) has more information if you’d like to explore this free source of dye adjunct further. There’s lots in this book about working with children and the textile arts, too.
There's so much conflicting evidence on this, now I'm curious if we can do some experiments. By 'we' I mean someone else because my only heat source right now is indoors.