Without seeing it is hard to suggest, but you could train some new and pliable growth to grow horizontaly or even slightly down, instead of up. Depending on how far apart they are, you may create greenery on one by directing new growth of another to bare spots.
Another possibility is to plant young one in between and train it from very beginning to grow horizontaly and slightly up.
I remember reading or hear somewhere that you may induce new growth from the woody trunks of H. petiolaris by wounding it, but I'm certainly not the one who did it myself and who would take any responsibility for such suggestion.

All sound like good suggestions. Here's one more.....
What about just cutting back a stem or two down to the base to encourage new growth from the bottom? This works with other hydrangeas but I have never tried it with Petiolaris. (wish mine were mature enough to have this problem) I agree with EGO in that I would approach with caution. Wouldn't want to do anything to adversly affect a beautiful climber. If your plants are that large, I don't think you would miss an inconspicuous stem or two, but without seeing it myself, you would have to be the judge. One more question....is the sun reaching the bottom growth? I wonder if it is possible that lack of sun could also be a source of the problem causing the plant to be "leggy" as others in shade do. Just a thought. Good luck in your new old home and with your bounty of mature Petiolaris. Sounds like you indeed hit the jackpot!!...yg

Thanks to both of you. That is very helpful. One of the lucky things is that there are four of them, each with a number of "trunks", so I think I will probably read up on the "wounding" technique, and also the idea of doing a hard prune. I would think that it already has an incredible root structure, so if anything could go nuts, it would be this one.

But I also think, especially if those tricks don't work, I'll plant gallon petiolaris at the base, and let them blend.

Since you already have blooming age vines I'd go for 1G solution as a sure and inexpensive thing.
Train it (them) while young and you'll have a cake and eat it too.

Simcan, if you into unorthodox experiments, you may want to plant young Schizophragma 'Moonlight' instead of petiolaris. They have identical cultural requirements and growth parameters, but have a differently looking leaves and flowers. I guess if you'll grow one thru another effect could be quite interesting.

Upon reflection I had pretty much decided to go with a few 1 gallon petiolaris to blend, but the Schizophragma 'Moonlight' suggestion is a very interesting one to me. Of course, I will probably do both...there is a lot of wall, and I was going to start a different thread somewhere for suggestions of good clinging vines other than petiolaris. Thanks again.