IF they are built safely like vortex - then yes Ewan.
A bar or big flywheel mounted onto a 10mm motor shaft with a M6 bolt and nut holding it on - is not safe mate. I would advise you to maybe send pictures of your construction of ur spinner to us or roaming robots before you turn up and get a dissapoiting Un safe stamp on your tech sheet.
Yes they will allow it at Robot wars, because they have spent alot of money on a near perfect safe arena, so they dont care what happens in the arena up to a point.

Mr Stu