Most syndicates I've been in limit to 2 or 3 the number of bookings anybody's allowed in the diary ahead of time. That pretty much solves the weekend-hog problem. One I'm currently in limits us all to a maximum half day each weekend as well - I don't particularly like that rule as I'm fond of being able to take the aeroplane away for a full day, but it also serves the purpose (and a couple of times I've just emailed the whole syndicate well ahead of time and got agreement anyhow).
G