I believe either in this thread or some other threads about the Priority Club lounge access - that enough people reported that they had to sign slips for bringing in guests. In the cases where the guest number exceeded the allowed number, they received a charge on their cards weeks later. So, without "Source", just by reading various threads, there are empirical data points to indicate that the clubs do have the ability to bill you back, albeit may not be directly - but the charge does show up on your related card.