You'll need to talk to the event organizer. It'll be on a case by case basis. There is no hard and fast rule on this sort of thing.
The lvo for example is very open with what sort of proxies/ conversions are allowed. Other events, might be more restrictive.
The most important thing, is that people should be able to immediately tell what each unit is supposed to represent by just looking at it. If people have to ask what things are, then you'll need to fix that.