Rand's choke looks like the same concept as the Featherfire choke. Only at a fixed setting (can't tell from the photos).
I've played with a variable choke on a chimney stove before; actually, it happened by accident when making a dual-wall chimney burner for my vaned kelly kettle...
I made the burner using techniques used in the 'Pepsi can burner'; i.e. a fuel cup base into which is inserted another can. Since I wanted to make a chimney burner, I punched holes in the outer can, and then punched mating holes in the inner can before assembling the two. Assembly proved to be tricky, so I ended up opening up the inner holes (since I didn't need a good seal, unlike in a jetted burner). This meant that the inner can was relatively loose, and could be rotated. Thus, the alignment of the two sets of holes could be changed, and, in doing so, the cross-section of the holes could be changed.
I can't remember the results...
I've suggested the same overlapping circles method for air control for wood stoves (to allow experimenting with primary and secondary airflows, and to 'bank' the primary air flow to reduce the heat output).