There is one other way to make an adjustable-length center pole. I use this for a low-slung tarp shelter, and it probably does not need the pole strength that you might be looking for. First of all, I started with a pole repair kit for tents. This kit has a number of hollow fiberglas tent pole segments with ferrules that are on the order of two feet long. These can be assembled with the center shock cord, and they can be "cut down" for length. The normal assembly method would probably use a number of these segments of approximately equal length. This is where we change it.
For example, use two full-length segments plus some short four-inch segments, and shock cord them all together so that the maximum assembly length might be 5 feet. Then, if you want to shorten it by four inches, you just pull the last four-inch segment out of one end and fold it over. If you want to shorten it by eight inches, you just pull the last two four-inch segments out of one end and fold over. The shock cord keeps the segments from getting lost on the ground.
You could probably do this with hollow carbon fiber pieces also, but you might have to look around to source the pieces.