My program HARMONY does let you explore the possibilities mentioned above, of different scales and fingerings, crossing the rows, and lets you find chords on the keyboard (just enter a tune containing only the notes of the chord).
I keep meaning to transcribe some of my own arrangements, but never find time, and am aware that they may not be possible on other people's D-G boxes. For example, my 1979 Hohner Erica does NOT have accidentals at the bass end of the keyboard, I think this was a later innovation. I rely on the bass notes for my Rosebud in June, where I play all the chords on the keyboard, including the Bm.
I arranged "Old Black Joe" from Bucknell Morris so that I could play along with the accordeon, using the same chords, which meant a certain amount of crossing of rows - but it sounds much better like that, in my opinion.