We don't have a function specifically for this (yet, it's coming). However you can use path's to get something similar. It's not a perfect solution, but it works.
The trick is to use a path with the following settings
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A single bone with a Spacing set to "Percent" with a value of 100 and also make sure it uses "Chain Scale". These settings will force a single bone to always span the entire width of the path. The path vertices are then weighted to additional bones and can be moved around separately to make the path stretch, which also forces the bone to stretch.
I hope it helps.
Attached is a project with the setup you're after, note project was made with latest beta.
stretchybone.spine