I don't follow the logic of not wanting to use a post when a tree is handy. As discussed above, using living trees as
fence posts to hold nails, staples or wire is poor design with respect to several factors.
Using living trees as hedges has some appeal but takes a while and once they've grown into a hedge it's difficult to see through them to what's going on in the next paddock. Live hedges are also liable to be colonised by wild mammals which may or may not be a good outcome. Foxes and
rabbits like a bit of cover, for example.
Wattle fences can be useful for some livestock but are labour intensive to make and also lack see-through-ability.
I'd like to have a crack at pleaching to see how strong it can be but I don't have any data at the moment.
WRT
sustainability, in my opinion a good solution is post and rail using durable timbers. In Australia, this might be a
Class 1 or 2 inground durability timber for the posts and a Class 2 above-ground for the rails. But this is labour and materials expensive and while it will hold
cattle and horses a fence of this nature may struggle with sheep, lambs and small calves. Putting an electrified wire at an appropriate height will help with this.
My understanding is that in traditional agricultural systems in Europe it was common to have someone handy to keep an eye on them for much of the time. So if animals did get through a pre-wire fence this could be dealt with before major damage to a crop etc But with labour costs under our current economic system it makes more sense to have a stronger fence. And can you see anyone these days showing a lot of interest in spending day-in, day-out keeping an eye on a herd of sheep?
You could try making a list of the objectives you're are trying to meet, prioritise them and then make a list of possible solutions with pros and cons. Match these up and a feasible, acceptable solution
should become apparent.
On a related note, I think a more structured design approach would help in a lot of areas in sustainability but that's another topic
Hope this helps
David