Varying units per em?

I’m working on a variable font with an xHeight axis. Everything works well, but it would be very handy if I the Units per Em could be specific for each master (so the leading would vary with the xHeight).
Is there a way to do it ?

I dont think the UPM is variable.

Ok, thank you. If by any chance somebody thinks about another way to make the leading change when the xHeight vary, I’m curious to know it!

I’m not really sure what you are trying to do. You might need to scale everything else.

This is how the xHeight vary :

When I change the UPM, it affects the leading (at least in html/css. In Illustrator, I think it’s calculated automatically, but it doesn’t change neither when xHeight varies).
This is how I wish the leading would react when varying the xHeight :

I don’t think this is possible. But it seems that you could do that by changing the Width, Weight and font size simultaneously to get to this result.

Ok, I expected that.
Thanks for your advice!