[Mark]
Ideally there could be a property `minimum-length-before-break` to match `minimum-length-after-break` to address this? Or am I simply approaching this problem from the wrong angle?
I don't think you're approaching it from the wrong angle. You need to get the springs & rods problem to express the constraints you want. I don't have the bandwidth to look into this right now, but a good place to start would be the implementation of ly:spanner::set-spacing-rods in lily/spanner.cc. [Kieren]
Does \alterBroken work? Or does it fail because minimum-length is one of the "spanner properties accessed before line-breaking such as direction" [known issue]?
Yeah, it is one of those. Line breaking makes use of knowledge about the horizontal spacing (there are more measures per line if they are short, for example), so those spacing constraints have to be accessed before line breaking.
