I've updated my spine launcher to 3.7.20 (it was using 3.6.49 previously). The scale option now appears to be available but it still isn't properly applying. The resulting skel binary looks as though no scaling has been applied.
Again I've tried this using both 3.6.53 and 3.7.19-beta. In the beta version I was seeing the same behaviour as before but amusingly using 3.6.53 the scale property appears to scale the UI? I also get an error, "Invalid Action: Audio View".
I was able to find a workaround by splitting my export into two calls. One to import from .json to .spine with scale then a second to export from .spine . skel. Really feels like this should be doable in one action though.