In point of fact, I don’t think the particular rounded triangles built by Instructables user sherrycayheyhey to show off this unconventional Lego technique are quite convex enough to be proper Reuleaux triangles. But the general idea—exploiting the small play in each of a large number of individual joints and, to a small extent, the flexibility of the bricks themselves—could probably be used to make very close approximations. You’d just have to make each side longer by the right amount.