Step 1: Chuck it up!
In my case, a 7/16-inch socket with a hex base made for hand drills had the perfect exterior diameter for a really tight friction fit with the interior diameter of the tube.
Sorry for the mixed-up photo timeline. I really came up with this quickly and was surprised it worked as well as it did and I didn't think to take pics until well into the project.
Step 2: Start Lathe-ing!
The hacksaw blade's teeth will need to be facing into the feed or rotation direction. The vertical - not angled - face of the teeth should face into the tube material as it is rotating in order to cut.
Start the drill slowly to get a line or indentation that will help "hold" the blade in place on the tube before getting all Western and pulling the trigger fully.