Putting a support under the branch actually makes it weaker, since the limb no longer needs to support it’s own weight. After enough time should the support come out from under the limb it’ll drop right to the ground and likely break off. Supporting the branch is still what you need to do, but there better ways to do it.
The problem with wrapping anything around the branch is girdling.
This not only risks killing the limb, but as the branch continues to grow it will try to grow around the loop. Eventually, as johnpost said, either the new growth will envelope the loop (looking something like this } { until the bark meets, this can take years) if the loop is loose enough. A tight loop is more likely to girdle the branch and kill it.
The better option is to attach the wire to a wood threaded eye screw in the trunk and a machine screw threaded eye bolt with a thick washer or a couple of thinner ones under the nut for the branch (stainless steel if you can get it). In the case of the branch you’ve got the largest diameter you can find, the longest length you can find. Finding the right length is gonna be your biggest problem, 10" or 12" would work fine for your branch, 8" might work too.
Drill a plumb hole through the branch, same diameter as the bolt, and a counterbore hole (preferably with a “flat-bottomed hole” drill bit like a spade, brad point, or Forstner) the size of the washer on the other side. The counterbore only needs to go through the bark, not much deeper. The counterbore hole in the bark should be elongated so it looks like this ﴾◦﴿ with the curved cuts meeting, like an eye. This is the orientation for a hole in a tree trunk, your “eye” should have its long axis in line with the length of the limb (the points of the wound point to the trunk and branch tips). I’m not remembering if you want to remove everything down to the wood or not, but that shaped wound heals much faster than a simple round one.
For the trunk side the largest diameter, longest length wood threaded eye screw you can find, the longer the better. It’s a whole lot easier to screw them in if you drill a properly sized pilot hole first. Leave an inch or so of the eye screw’s shank visible, room for the tree to grow and still replace the eye screw if necessary (the screw-eye in the limb, not so much).
Steel cable is probably the best choice to run between the eyes, chain would work too. At first it will be under tension because you’ll most likely be pulling the branch up, as the branch grows it’s the weight of the branch that the wire must carry. So chose the wire size real carefully, basically bigger is better. Remember you need to support the limb and any snow or wind loading, you’ll probably want something thicker than you think you need. Push the limb up to a little higher than your desired height with a 2x4 attach the wire and your done.
This is very similar to the way trees with twin leaders (one tree, two trunks pretty much the same size) are repaired when they crack at the fork. You drill two holes through the trunks and use a steel rod to pull the trunks together.
CMC fnord!
