I managed to get to the circuit board inside the motor. Attach1, subject to forum not rejecting my photo.
Further disassembly is invasive and destructive too. The 'stator' core pack hides the board under it, and has been fitted into position on a hydraulic press.
Probing where I can, I see that the main +feed is interrupted by a blown SMD fuse, which presumably should kill all circuit functions. Probably about 10A, can't see.
Temporarily feeding +15V to the load side of the blown fuse draws 20mA or so, and, more importantly, sets up the 5V reg as detected on the violet control wire. I dare not increase the input further, the motor is still disassembled, no rotor. This tells me that simply replacing the fuse (somehow, in situ!) gives me a sporting chance of repairing the blower.
So I wired up the second unit to a controlled and limited supply. There are no mysteries at all: starts up at 16V and goes to highest speed directly as limited by the voltage. At the rated 24V, it draws the full 4.5A, and, boy, does it kick up a racket! The control wire needs just a pot to ground to pull the speed down, and works to very low speed. No doubt PWM at any frequency will work too, not tried. So far so good.
'Hot plugging' feature just disconnects or connects the power ground through a loop within the motor, so two wires, blue and black.
My thanks to all who responded.
Edit, 4hrs later: Well, what do you think! Alas, I couldn't get an SMD fuse, but had a PTC self-resetting job rated 5A, which could fit snugly. I did not remove the blown SMD, but shunted it by the PTC which had just the right lead spacing to solder on. No prize offered for the right guess, but it is working perfectly now!