The designs you have found are for trees, notably fruit trees, which will have a leader and side branches. You cut off the leader to encourage side branches and then select a new leader to continue upwards. Once trained (a process which can take several years) the tree will form a permanent framework which will then have to kept pruned into shape at least annually.

Climbing Hydrangea (by which I am assuming you mean H anomola sp. petiolaris) does not behave like a tree. If you cut off the central leader it will branch out but not necessarily neatly and not necessarily providing you with a suitable new leader. It is a pretty messy grower and trying to force it into a rigid shape is unlikely to work. It would be much better allowed to make its own way up a trellis with judicious guidance.

It is a self clinging climber, like English Ivy, which attaches itself to a wall or tree by little rootlets or 'holdfasts'. It needs something permanent and solid to stick to as it does not have tendrils or a twining habit. But you seem reluctant to build a proper trellis. It does not lie flat to the background either but puts out branches which you would constantly need to trim back. It is also quite a slow grower and would take a few years to cover your space. But once it is established it can end up at 50ft high.

I sound as if I am pouring cold water on your ideas again but this one , as you envisage it, is I am afraid, a project which a highly experienced gardener would regard as a challenge.

The more I read your queries the more I think you need to build a robust permanent trellis and use something like Virginia creeper or even English ivy which will lie flat and not flower.

Alternatively consider annual climbers grown in a row between the posts and trained up strings.

flora - cimbing hydrangea can indeed be made to look something like the image a. above. I'm sure it takes a far bit of work though! My climbing hydrangea on a fence is definitely allowed to be a messy tangle :-) But this is what one at the nearby Royal Botanical Gardens looks like along part of the pergola walk:

But I agree that it might be easier to use a fruit tree to give the same effect. Here is another picture from the pergola walk - this one is a fruit tree (apple I think but can't remember for sure...)

So there's more than one way to skin the cat :-)

I agree with you woodyoak - it certainly can be done. Skilled gardeners with knowledge, patience and resources can get plants to do all sorts of things. I was just trying to point out that it is not an easy job as it goes against the natural habit of the plant and requires a well built trellis and some expertise. The one in your picture must be tied in regularly since the holdfasts cannot attach themselves to wire. Not sure if you've been following this saga, but the OP has been asking about making a plant wall on the Clematis and Vines forums and is, I believe, a newbie gardener judging by the questions. The designs in the pictures would tax an expert. He wants a non-flowering vine. He also doesn't seem to want to build a robust permanent framework, even strained heavy gauge wires, and without it I really don't think the Hydrangea idea would work. I just felt that the designs were over ambitious and would not turn out like the picture he had in his in his mind's eye.

flora - I understand your points and agree with them in general. But I lean towards encouraging people to try things that interest them, even if they are new at it. It's easier sometimes to figure out if whatever-it-is is something you can manage to do if you see and example of it in person and can examine it closely to see how it is done. Given the OP's location is Ontario, the RBG pergola walk is a great resource to help him/her (?) Hence my question whether it is reasonably close enough to visit. I think espalier takes a person with an organized and precise frame of mind to do - that's not me! But it might be the OP - you never know... :-)