I've never heard of that here. Some people have tail hairs made into a bracelet, or keep the last set of shoes for sentimental reasons, but that's as far as it goes.One other little tidbit that may prove useful to you. At least in the States with dressage riders, it is relatively common, when a primary showhorse dies, to cut off the tail after the bone and preserve it as a memorial. My mom has St. Just Juliet's and Rising Eagle's tails displayed, along with a framed photo and most of the ribbons and awards they won on their own walls.
Both your mum's horses are beauties. Here are some of ours. The appaloosa is a Danish Warmblood cross (Knabstrupper x), and she's the one who broke Katrina's pelvis. The liver chestnut is the ex racer (he'd been dragged out of retirement for this photo), and the black mare is a Welsh cob x Hackney as a baby, at her first competition. She had her tongue over the bit the whole way through the test, little brat. Learned that trick in the warm-up and raising the bit didn't work!
Deep apologies to @Montero for hijacking your perfectly good questions!