"Freeze the rig, yes, this a bit the idea, it depends on the details. Here, the script copies the K4 JCMs and blends them. There is no scaling involved, I was thinking that scaling is the same as animating the joint."

Well, the results of scaling and animated joints used to be very different. Not sure if the revised scaling is still different on a technical level. It still seems actually moving joint centers around achieves better results than scaling bodyparts. Not to mention that if you scale a leg along the y-axis, the knee gets elongated, too, while if you morph the legs longer and re-adjust the joint centers, the knees can be kept in the correct shape.

In any case, all what would be needed is a fully scaleable basic rig where the scaling can be permanetly frozen. JCMs are mesh specific, so they would be of no use anyway (Unless the new figure was made of the exact same mesh as the old one).

I'm thinking more of an universal rig that would prevent the need to create a rig from scratch in the SetUp room, or to exactly match the new figure to an existing older one.

Yes, Studio can "auto-rig", although I dont know how good the results really are.

But I'd already be very happy with a way to eliminate all the scaling out of a modified figure and make the proportions permanent.