All CLA requirements met.

Additional application idea:

• make ligatures properly respond to spacing

Perhaps also mention latin-based connected script typefaces.

I am interpreting this proposal as for 'optical spacing', i.e. it isn't simply an x-direction 'white space' adjustment feature (although could be implemented as such), but may also involve adjustments to glyph shape (indeed, may require such adjustments in the case of joining scripts), and may involve variable GPOS interaction, e.g. to scale kerning proportionally to variable spacing. @LetterModeller does that interpretation fit with what you have in mind.

[I think that for all new axis proposals we need to understand whether the proposed axis is optical or parametric (perhaps this is a category to add to the standard proposal template?). I define an optical axis as one that may affect more than one aspect of a design at a time, e.g. stem weight changing on a width axis; whereas a parametric axis affects only one aspect at a time, independently of others (cf. Amstelvar). These represent two different models of designing a variable font, and it seems fairly important to distinguish whether a given axis proposal is optical or parametric. All five of the initially registered OT variations axes are optical axes, although only one — Optical Size — explicitly acknowledges this.]