Split chord across staves
This is an important convenience for two reasons:
If a (> 4 note) chord is split into separate chords (> 2 note) across staves, first follow the workaround at: How to span a stem over two staves. Because the fixed point of the stem is the upper note of the lower chord, and you've dragged the previously lower end of the stem above the fixed end, the lower note of the lower chord is now stemless. Select the stem, and change the vertical offset to remedy this. It can be difficult to get the exact distance - otherwise, a notch from the extended stem may show.
playback with arpeggiato (or other articulations):
To arpeggiate across staves, write the entire chord in the upper staff. Mark the notes you intend to move to the lower staff as invisible (but keep them playable). Inscribe the lower split chord into the lower staff, and mark the notes as non-playing (but keep them visible). Arpeggiate the upper chord, then edit the arpeggiato symbol to extend across both staves.
So, clearly it would be greatly convenient to have natural support for split-staff chords.