Hi Dan,
Not as creative as you may think.
Essentially Lilipond as Tex is about stacking boxes with some rubber space/bands in between.
If you read Chapter 11 "Boxes" of TeXbook (Donald Knuth) you will understand what I am talking about.
What I did is "add a box on top of the tallest box in the row", thus forcing the row to be taller still.
Tex has \vskip, LaTeX has \vspace ... It seems Lilypond has nothing.
As good as Lilypond is (and it's great), it will never be perfect. There is a reason why TeX and LaTex have custom vertical space commands.
Obvious hint: It is needed.
And after some very extensive search, I found this problem resurfacing several times. In my not-so-humble-opinion :D
It is needed!
It looks like there is no proper solution, only various hacks. I'm giving up. :(
Thanks to all who replied :)
All the best.