For what it's worth, we had 2 subarus, a pulsar and I think it was an ASTRA of all things, on the Japfest2 stand. They all sat down the bottom of the stand out of the way and the Z stand integrity was not harmed At the end of the day, the stand is organised by us for Zs, but if we can't fill it up with Zs for whatever reason (People going on trade stands, show n shine, engines blown up, realised they're on holiday, etc.) then a little bit of stand filling isn't going to hurt anyone.
In principal though I agree, the stand is primarily for Zs and this is why we try to keep tight control over who has stand passes.