Actually, yes, hacks from
BL and certain dumb
RPG system say you can maglock it to leg, but in reality, it would be comically stupid idea. One, magnets are really weak in perpendicular direction, so on something moving, like a leg, it would constantly slide around just from inertia scraping both armor and the weapon surface (and would readily fly off if weapon caught on something). If you look at actual models, you will always find weapons locked to backpacks or armor butt plates, areas that don't move that much and allow steady lock (example):
https://www.games-workshop.com/en-US/legiones-astartes-mk4-tactical-squad-2022
The only way you could safely lock your weapon on a leg area is on armor marks with separate thigh plate that is mounted to armor belt/torso, instead of leg, as that arrangement moves much less and doesn't throw your weapon off on the first bush (in much the same way scabbards are tied to your belt, not around your leg, it just doesn't work). Which, funnily enough, is only readily found on Primaris armor and is yet another example why Mk X is superior and makes far more sense than old, fugly, wonky armor marks so badly designed they would only allow slow, comical waddle in real life (if that, because their armor plates tend to clip through each other even on 'taking a dump' standing still poses in old models)
Here, example with helmet and sword locked to belt/plate, any other mark of armor and stuff maglocked into legs flies off even on a sudden stop or turn, or worse, swivels around the (round) leg armor and your own sword stabs you into the groin unless you maglock it to your leg so strongly it will first lock your legs together then bend you over and lock legs to your torso