I realized that you might be able to sort of do this by turning off automatic note detection and playing the lower note whose partial you want to tune to, for example set note to C6 then play F4 to measure the 3rd partial... but it doesn't seem to work reliably. When I play C6 first then play F4 immediately after it does seem to give a measurement. But if I then set the pitch to C#6 and immediately play F#4, it doesn't register anything, like it's not detecting the note it's looking for so there's no display.
Here is my idea of how this feature might be implemented in PianoScope:
1) play the note you're tuning, automatic note selection selects note
2) hold down a button "tune to partial"
3) while holding "tune to partial", play the lower note whose partial you want to tune to
4) once the pitch registers on the display, release "tune to partial". the button changes state, and an arrow indicator appears on the scale as the new tuning target (similar to pitch raise mode)
5) when done tuning the note, press the button again to return to normal mode.
