Yep!I love the sash idea! Would make workshop images rather obvious, although perhaps a little difficult to get a kingfisher to wear one!
The problem with any competition is cheating and I’m going to come right out and say that any and every workshop image is the result of another photographer regardless if you think you chose the shutter speed because let’s be honest if you got the shutter speed wrong somebody would be along to help you. It’s painting by numbers.
With that in mind no they shouldn’t be used in camera club comps but policing that is too big a job.