To be clear about the nomenclature, yes if you are not using an ESC or servo to control turret rotation (for example you are just driving the turret rotation motor directly from the TCB as most people would), then the turret rotation RC output (output #3 on the board) becomes available as a pass-through signal from your radio. And then yes you can assign any channel on your radio to that output and you can hook any RC device to that output you want, including an RC switch.
The same holds true for most other RC outputs. RC output #5 is the exception, it is always reserved for a recoil servo whether you use one or not. But the other outputs may be free depending on how you control other motors (outputs 1-4) or what sound card you are using (outputs 6-8). Most people will find themselves with several RC outputs free for general use, or "pass-through" from their receiver. Of course if your receiver has PWM channel outputs you can just use those directly, but many small SBus/iBus receivers only have the serial connection and no individual servo outputs, so the "pass-through" feature of the TCB can be useful in those cases.
Now - if all you need to do is turn something on or off, it may be more simple just to use one of the various switched outputs on the TCB rather than a receiver controlled switch. We already mentioned the smoker output (if you are not using a smoker), but there is the
general purpose Aux output as well as the
four light outputs Light 1, Light 2, Brake, and MG. Although they are called "light" outputs you can switch anything you want from them so long as what you're switching accepts 5 volts and doesn't draw too much current.
If you need to switch something on/off that requires a higher voltage or current, you can also connect a relay to the Aux output or either one of the two
general purpose I/O ports. You can buy cheap relay modules on eBay that are plug-and-play with the I/O ports as described on that page.
TLDR; yes a receiver controlled switch is no problem! But you have lots of options.