Outdoor ceremony logistics - getting to start of aisle!

I don't think it matters if people see you before you get to the top of the aisle. In a large indoor venue, the people at the back will see the bride long before the ones at the front, so it's not as if everyone sees the bride at the same time in indoor weddings. I'd maybe try to get any 'before ceremony' photos out of sight of the ceremony location, so you're not standing around posing while everyone is watching, but otherwise, it's not a problem if they see you walking toward the aisle.

I'm not sure telling everyone to face forward would work anyway - there will always be a few who won't be able to resist turning round!

Telling everyone to face forward will not work. Yes almost everyone will be looking back at you just like they do when you reach the isle. Embrace it and treat it as an extra long isle and the groom should definitely be looking back at you, its a view he will never forget rather than the first time he sees you - standing right next to him where he can not properly see you. here's one and i think everyone has turned around. And the last shot shows how far the walk was.