No, this is not legal. 3D printed parts are not part of the VEX EDR product line, thus they are not legal unless specifically allowed in <R7>.
Let’s take a look to see where 3D printed parts for cable management could apply.
Since 3D printed parts would not have the same composition as any VEX EDR parts, they can never be considered identical to any VEX EDR parts. Thus, 3D printed parts are never considered legal identical replacements.
3D printed parts may be legally used as non functional decorations, however since these parts would be used for cable management, they would be functional, thus illegal.
Since your 3D printed parts are not commercially available, they would not be legal.
In summary, it is not legal to use 3D printed parts for any purpose other than as non functional decorations.