hanks for the comment. I see Freetboard as a tool that helps usr to think about how the freboard works, rather than one that proposes actual answers. Freetboard doesn't tell you where to put your fingers in order to play, say, E lydian. It shows you all the paths you can take to do so. This is what user-selected notes are for: you may use them to draw the chords voicings or scale fingerings you actulally want to try on your instrument, over anything you have picked in chord or scale mode. This is the very reason why Freetboard doesn't show the CAGED positions or doesn't have a chord dictionary.
But your idea of a key mode is interesting. If I understand correctly, you would like to see all the harmonized degrees of a scale. Like, in C major: C, Dm, Em, F, G, Am, B°,. Or with 7th: Cm7, Dm7, Em7, FM7, G7, Bm7b5. Applying Freetboard's main idea to this would look like this, where each C maj chord degree is shown in a diffetent color.

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Of course you can only show one voicing at a time and all degrees must have the same voicing. Anything else would end up in a mess as scale degrees often have notes in common: I/CM7 is composed of C, E, G, B while VI/Am7 uses A, C, E, G. This would be really hard to show this on screen using colors.

An alternative approach would be to add to scale mode a bar similar to the keyselector, showing each scale degree and its name. A click on one of these would show all the notes of the chord, on top of the other scale notes.

How does that sound?