Some lamé fabrics are woven and those mostly fray just by looking at them and would be very difficult to work with. Other lamé fabrics are knitted/jersey. They don't fray badly and will have a bit of stretch, but because the glitter is printed onto those fabrics they need to have a very gentle wash, unlike denim which takes any amount of machine washing.
If you are thinking of doing an appliqué patch you could use a bit of Bondaweb, Mistyfuse or similar to hold the patch in place and then a zig-zag or overcast machine stitch all around the edge of the patch to make it secure. Or you could work blanket stitch by hand all around; that would mean that you wouldn't have to open up the leg seam.
A patch on the shin of one leg balanced by another on the thigh of the other leg would make it all look more intentional.