I think "braconné" is an unusual usage in this context. I imagine it means what is usually referred to as "treillis", a cross-bracing of steel members. Power pylons are said to be "lattice pylons", the lattice being a form of bracing.

"Bracon" is usually said of more "isolated" bracing, individual, non repetitive. Thus, some bridges have "wings" projecting from the side of the main part of the deck, these wings being supported by diagonal braces or struts known as "bracons". Though the "bracons" ARE repeated, they project individually from the the main deck section, are all aligned parallel to each other as a rule (unlike 3D space frames for example, which might be said to be latticed, "à treillis".

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Note added at 1 hr (2007-12-28 14:23:55 GMT)
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I've just looked through an EDF guide to power pylons. Nowhere do I find "braconné". Lots of "treillis" and "triangulation" (lattice) and "haubané" (stayed/guyed), however.

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Note added at 1 hr (2007-12-28 14:31:24 GMT)
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I think this must be North African French. The first two relevant sites for "mât OR pylône + braconné" were Tunsian and Algerian:

http://www.msgi.com.tn/fr/produits.php?id_cat=2&id_parent=7
http://www.acg-dz.net/index.htm

bracon - Etai incliné soutenant par exemple un encorbellement et formant contrefiche. Voir aussi "Bielle".
[Dicto TP, Editions Arcature]

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Note added at 4 hrs (2007-12-28 17:14:16 GMT)
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To Mediamatrix: No, there are no rigid bars replacing stays (not external to the outer envelope of the mast anyway). The pictures on the MSGi site above do not show this, certainly, but rather three-legged towers with a minimal amount of (rather robust) cross-bracing between the 3 uprights. Funnily, where the French says "Mâts braconnés et sur façade", the English merely says "roof poles"!

In structural terms, the cross bracing between the uprights DOES replace external stays/guys.

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Note added at 4 hrs (2007-12-28 17:26:33 GMT)
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Admittedly, the "bracons" on this trailer-mounted mobile mast ( http://www.freepatentsonline.com/EP0296957.html )ARE "external to" the mast proper, acting like stays/guys. And some of the photos in the second site above (ACG) do appear to have two "legs" on one side of the mast, so indeed, "braced" would be a good coverall word.