Staff Size for keyboard top staff only?

Is there a way to scale only ONE of the staves of a keyboard, or other grand staff? E.g. Editorial realisation of a Figured bass, so right-hand at reduced size? Using Staff Size changes both.

I could just use separate instruments with a Brace applied; though no easy way to put the staff label in the middle…

This won’t work if you have other instruments that need labels too, but will work if you only need a grand staff. In Setup add a single staff instrument, label it whatever you want, and duplicate it. In Engrave change to a brace and scale the RH to whatever you want. In Paragraph Styles, set Staff Labels (Inner) to 0pt to get rid of the 1 and 2 on the systems. Fix the Staff Label positioning in Engraving Options for the remaining centered label if needed.