1) limit the heights at the broken edges so the height/width ratio of
the sibling doesn't exceed a certain value (I'd start with 1/2)
2) make that ratio and the broken-heights values user-settable as grob
I've given it a partial try that can be seen at
https://github.com/lilypond/lilypond/pull/4. This commit (hopefully)
implements part 1) of the above. I've also attached the patch. Since I
don't have a working build environment right now I couldn't test this
code, but it should be straightforward, I think.
I would be glad if someone could add the code to create the grob
properties (should be little work but not if you have to first look
everything up like me) in order to upload it for review later.