I have though about something like this for some time but never got to it (the RMX-tools are so handy that I never needed it 
).
But I just had an idea. The smart components can do all what’s needed. Just the UI is missing. But with the help of some scripting, I could set it up:
In the Macro Window, run this with the “two” glyph active in edit view:
part = GSPartProperty.alloc().initWithDict_({"bottomValue": 40,"name": "Weight", "topValue":100})
Layer.parent.setPartsSettings_([part])
Then run this with the lightest layer active:
Layer.setPartSelection_({"Weight":0})
And this with the heaviest layer:
Layer.setPartSelection_({"Weight":1})
Then in the glyph with the component to the two, select the component and you should see a slider in the lower right.