I think the way that JY currently manages the holidays works fine. From what i have read, very few holidays get cancelled. Whilst it might be a good idea to promote it on the website when a holiday is about to be closed, I think to have that information in the forum would be a waste of time, as there are far more people who book holidays and do not using the forum.
Personally I have to save for each holiday I take as I don't have much disposable income - so I book in advance so I have time to save. For those people who choose to book late, one would assume that the money is not so much of an issue.
Perhaps a better option would be to indicate on the site or in the brochure the minimum numbers needed for each holiday...that way when people who use the forum ask Julia about numbers - they would already have an indication of what is needed for the holiday to go ahead.