I expect building a script to make this happen would be fairly straightforward (and I anticipate that I’ll write it), but thought that a potentially useful feature would be to enable relative spacing across masters.
So in the light, n to n has spacing of 20, so in the bold, you could say light n + 20 or such and it would keep everything in sync. Of course, doing these things individually or on a per weight basis would be ever present, but this kind of automation could be handy.
Only as a starting point though.
Like all features, mileage may vary.
I would expect that for certain designs it could be a total package. For others, not.
Your bold sidebearings are wider than in the light?
It is primarily as an example more than a “here’s what I’m doing”. Imagine an optical style instead. The caption style should have more space between letters than the display, right? So handy to have them kept in line.
If you give me a good way on how to indicate in the metrics key that you target a different master, I’ll see what I can do.
Aren’t you afraid this can become super complex? As in, ‘help, I am losing oversight’?
I wonder whether a script shouldn’t be doing this, and store values in the font. You could use a general value for the master (e.g. +10 for everything), and put exceptions in the glyph note (e.g. +15 for g).
My request was implying that complexity. But if someone has an idea how to solve that…
How about =glyph[MasterName]+value
=[MasterName]+value can even assume it points to the same glyph.
Or percentages of a mastervalue 
Yeah, say I want to set my left sidebearing of the Bold ‘n’ to 76% of the right sidebearing of the Light ‘z’ (apologies for the outlandish example), that could be something like:
=|z[Light]*0.76
not bad.