[ctlin] , [midiin] and [sysexin] are each passive receivers of MIDI data that must be actively sent by a MIDI device. Is there a way for Pd to poll a MIDI device? For example, read a Korg Nanokontrol's knob positions without changing the knob positions? Is there a way to read a "control" value, not just the "Control Change" value when a control's value is changed?

I think, some devices can dump their values over sysex.
So you have to find out the right sysex command by rtfm.

So Pd sends some Sysex message to the device, which causes the device to dump all of its controllers' status back over Sysex to Pd. Pd then parses the status response to find the specific controller(s) value it needs. Right?

yepp!