I think there should be a rule in all shows. When the thin-out point happens at the end of every day and on the last day, all bets are off.
The problem with the plinky-plonky dinner jazz played so extensively at shows is it does unfortunately fill rooms. I don't know why - when I've asked people about this, very few people actively like this kind of music - but if you reach too far out of band, the room empties. At the last two Manchester Sound & Vision shows in the UK, I handed the musical choice into the hands of the listeners, with a hard drive with hundreds of albums in all genre. There were some interesting choices (as I didn't have anything apart from magazine marketing and some subscriptions to sell, the number of people on seats was comparatively unimportant) but many people actively chose Norah Jones, SRV, etc, etc.
I can understand the reticence of manufacturers to play your music during the busy periods. Even if you pitched up with a piece of music that filled the room with glory, it sets a precedent that invariably leads to someone handing the manufacturer a true horror room clearing disc. However, my take on this is if you hear something you like and want to hear it with your own discs, ask them if you can come back when things are a bit quieter for a more thorough listen. That way, you get to listen to your music for longer and the manufacturer doesn't end up with the musical equivalent of a bad smell right at the point when custom is at its peak.
However, if the manufacturer flatly refuses to play your music under any condition (assuming, of course, they have provision to play what you want... no point getting upset about a vinyl-only system being unable to play your USB thumb drive) or at any time, I'd be reluctant to shortlist that manufacturer's products.