>> * Hot-swapping keyboards in X does *not* work, with or without wsmux;
>>   the keyboard appears unresponsive, although the hardware Caps Lock
>>   light still toggles.  Same thing when I add a second keyboard while
>>   using wsmux (but the first one continues working).
>
> Do you which device does the X server opens to get keyboard events?

I changed my configuration to:

    Option	"Protocol"	"wskbd"
    Option	"Device"	"/dev/wskbd"

So, it's using the wsmux device.

Likewise, the mouse configuration is:

    Option	"Protocol"	"wsmouse"
    Option	"Device"	"/dev/wsmouse"

and it works fine.

I think this may have something to do with the mode that wskbd is in
(delivering cooked or raw keyboard codes), but I'm unclear on that at
the moment.