I had similar concerns a while back before I put a Rock River CMP float tube on my Bushmaster. To answer your questions, the float tube is a solid pipe with the exception of a 3 to 4 inch slot cut in the top near the front sight block to clear the gas tube. The RRA float tube has three pieces. First there is a replacement barrel nut, this installs and provides a threaded collar for the float tube to screw on to. The final piece is a small lock ring that tightens against the new barrel nut and keeps the float tube from rotating and cinches up the threads so it doesn't droop.
The float tube itself is a great heat conductor. Standard handguards come with a heat shield installed that prevents air flow through the handguards. I don't think that the float tube would be any worse in that regard. It also has much more mass than the aluminum heat shields so it is a better heat sink. If I was planning on doing a lot of mag dumps I might ventilate the float tube with a series of small holes on the top side to allow some air flow.
All that being said, air is not a good thermal conductor, and unless the barrel is starting to glow, you don't get much heat dissipation by radiation. This pretty much means that the barrel is going to get damn hot no matter what. You can add mass to the barrel to slow heat rise or provide an efficient heatsink (one that makes contact with the barrel) to remove heat. Water jackets don't work too well on small arms so a heavier barrel is the best option. Something I haven't seen that might help is to put clamp-on aluminum heat sinks on the barrel, under the hand guards. This wouldn't add as much weight as a heavier barrel and with cooling fins, like you see on amplifiers, it could help dissipate heat by increasing the surface area.