I know it's lame. There are clubs that do it though. None of the ones mentioned in my first post, but some. There was a club around here that never advertised their trial and it wasn't up on the governing bodies website. Then when people did find out about it, the trial was "full" and they were charging if I remember correctly(this was last year) $75 per spectator. That kinda tells me they didn't want any outsiders for some reason. This has happened twice on the last year.
The majority of groups I can think of, are as you said. They advertise way early and try and get as many outside entries as possible. Heck my old club would waive the entry fee for club members if they had enough outside entries to cover the trial. Believe me, we wanted outside entries. My current groups trials are filled a month or two in advance. Most of those are outside entries. Actually last trial we had to turn it into a two day trial to get everyone through.