[Johnny .45]

Well, I suspect the "Control linkage" is just for elevator or rudder control, as it's location and the term "control linkage" are exactly the same in 90% of the cutaways in the book I have. Usually if if a cable for something unique, like an arrestor hook, it'll specify that it's for the arrestor hook, you know? That and I've found that one can't take these illustrations too literally. Usually they're pretty close when it comes to locations of things (not always though), but as for detail, they can be lacking in accuracy, especially with the more obscure aircraft. Hard to draw the details when there are no examples existent or accessible to the illustrator I guess! You have things like how many of the guns look strongly like Browning .50cals! Type 99/MG FF/MG51 ought to be tubular, the Hispano's receiver isn't much wider than the breech, yet most of them are your typical "box with a barrel" in many drawings. To be fair, many drawings are extremely accurate. In any case, I don't think they show the gun controls on this cutaway (this is the one I mentioned that I'd found on this site).
After I'd posted my question, I was reading about the "stinger" installation on the Do 217, and it was described as "remotely fired from the cockpit". It made me wonder if perhaps "remote control" really means "remotely fired". That's certainly possible, as I've seen installations like that on other aircraft like the Do 217. In all these cases, the gun is fixed and simply acts as a (slight) deterrent to attacks from the rear. That seems more plausible than an actual remote-control installation to me, especially as the stinger was something that was included on all production Ki-21's, which were introduced before the war even started. If the US and Britain and Germany had difficulties getting remotely-aimed guns to work properly DURING the war, it's hard to imagine the Japanese successfully putting a system into use before the war. If nothing else, the costs and weight would have been prohibitive in the Japanese way of thinking, especially for the potential gains.
I do have to say though, the fabric shrouding around it DOES suggest a movable installation, and in the (quite excellent) photo you linked to, it does LOOK like some sort of moving gun mount. If it is, your description could be correct; I'd guess a simple system set up with electrical switches (such as they used in interrupters), so it covers the area directly behind the tail. It may be that the stinger is set up so whenever the dorsal gun interrupter kicks in, it OPENS the firing circuit to the tail gun. When the circuit is opened, the gun automatically moves to the same azimuth and traverse as the dorsal gun is aimed and fires (while moving wherever the dorsal gun is aimed). As soon as the dorsal gun interrupters shut off, the tail gun is disabled. Or it may constantly follow the dorsal gun's azimuth and traverse (within it's limits of travel), so it wouldn't have to return to zero and back again if the gunner was forced to swing from one side of the tail to the other repeatedly. This way it's "ready" when the dorsal gun moves into the tail zone, and the interrupters simply enable it to fire. This sounds complex (and is!), but it's NOTHING compared to later-war fire control. Boggles my mind, that stuff.
Just seems like if it was that simple to do cheaply and effectively, that others would have used similar systems as well. And you'd think that it would warrant more mention when people write about the Ki-21 nowadays! It'd be far more advanced than any other aerial gunnery system put into production before the war, even if it's pretty basic compared to later Allied computer-controlled remote systems.
So try as I may, I can't answer my own question! I'd still love to know for sure, and to find out how it worked, if it did in fact exist.