Morten - you are 100% correct. As Centurion is an add on to the Legion standards, it still has to be Legion approvable in the end. Flubs such as duct tape, crappy paint, etc. may be OK from a prop replication standpoint, but aren't as noted what was intended and not what one sees on screen when things are moving, and thus not allowed on a Legion approvable costume.
However...in informal events you can do as you wish as long as it falls within your garrison's guidelines. For instance at a convention...