This is very interesting:
(Red Bull Rubik’s Cube Tournament Rules)
Article 8. Cubes
8.2. Cubes may be enhanced and modified as long as the design of the Rubik’s Cube remains and is comparable
to the provided Standard Rubik’s Speedcubes.
8.2.1. Alternatives to colored tiles, stickers or colored plastic pieces are allowed. Color schemes may differ from
the Standard Rubik’s Speedcube, as long as every side has one uniform and unique color.
8.2.2. Sides of 50.0 mm to 58.00 mm are allowed, as long as all sides are of equal length.
Maybe the use of Rubik's Brand cubes isn't mandatory after all?