Hanging from a power line you should be as safe as a bird.
The voltage difference is between the lines (e.g. in a 3-phase system) and between the line and ground. This voltage difference exists across the insulators and pole, as well as through the air to ground. These voltage differences are obviously small enough to avoid striking an arc, hence no current flows between the lines or between line and ground. If you are hanging from one line, there is no change in the separation between the lines (unless you are swinging wildly) and hence again no current flows between the lines. As the distance between the lines will usually be smaller than that between your feet and the ground, again no current will flow, and you will be safe. Note that, if this distance were too small, you would not be safe standing under the line either!
Your real problem will be to get down from the line. Unless someone can switch of the power, you will need to touch both the line and the pole. If the pole is wood you should be OK, but a metal mast will kill you as you will then have a path for the current to flow (from the wire, through your body and the mast).