I know a reference to something is normally WAY smaller than the thing itself, but I don't know how POL handles references to its objects.

So, is there any problem of high memory usage related to storing object references in custom properties?

I'm implementing a small chat system that uses a global property that is a dictionary mapping room names to array of player references. That is why I'm asking the question.

You should store the player serials instead and use SystemFindObjectBySerial to get the references again.

As a general rule you should never store references in a prop. Do exactly as Tomi suggests with storing serials. This is because props and references don't play nice with eachother.