StoreShapeKey is the base command used by save I think, and that should properly return the key. Bear in mind it will probably not apply it through the mixer, which if you aren't careful with the stack can become confusing. Apply it yourself, or clean after your script in those regards if you use it.
Worth reporting SaveShapeKey as mis-documented AND not working as intended, since there is no valid reason for it not to return anything of use.
