Thanks for the reply Pascal.
After playing around with annotation scaling in Rhino and comparing it to AutoCAD I can see that there probably is no way to get the best of both worlds. What AutoCAD is doing (I think) is creating a different annotation for each viewport scale but only showing them on a viewport by viewport basis. The upside of that approach is that you can adjust position, etc for each item (dimension, text, leader, whatever) without messing up the item in other viewports because it’s not the same item. The downside is that it requires quite a bit of maintenance. You have to set each scale individually.
Rhino’s approach, which I prefer, is the exact opposite: there’s only one annotation and it’s set universally for all scales or no scales - zoom in and out and the annotation adjusts automatically. But there is a downside to this which is shown in the two attached JPGs. The leader can’t be correctly positioned for both viewports (details). The first JPG shows the text in good position. The 2nd has the text too far away but if I adjust the position in the 2nd it will make the first look too close to the object.
Before the time of annotation scaling, one approach to this problem was to have different sized annotations on different layers that were turned off or on per viewport (detail). I think harnessing Rhino’s universal setting to maximum effect might mean resorting to this approach in some circumstances.