Black Friday Deal got me :)

also add the Longworth chuck to your list
www.carbatec.com.au/article/266/how-to-make-a-longworth-chuck
Not sure if it does square and rectangles

A couple of photos of Joey's setup.
He's running Qtplasmac. The rotary is attached to the gantry. I think he must use another unpowered chuck to hold the other end.
you can see his table is not exactly a hobby one

   

i was thinking Pneumatic clamping of some sort that could be controlled by Linux CNC. i was wondering if something simple like regulating air pressure between two independently could be a simple solution for calibration of the center line ?

just some more progress on the Z axis

what i was thinking was if you had a way to control clamping independently and had the pass through requirements you could have a tube feeder and not need a 24 foot long machine to work with a full length of tube. space saving in a good way i think.

been looking into this and i think it can be done but it is not cheap project off the shelf options are either through the roof in price or yet to be discovered. even making the stuff myself is going to be expensive by my standards and could see costing around $400 per chuck. So that being said i think i will start with a rotary union to see if i can pull that off and go from there without that the whole idea goes out the window

You could make up some machined blocks for your 4 jaw to hold rectangular tube centrally. eg 3" x 2" would need 2 x 1/2" blocks to make it a square.