I think these would really be nice additions. Funny thing is that this actually implies two options per each behavior:
As regards the Glyph Window behavior, when you move a node, handle, anchor, component in one master will move it in other masters too. There,
(1) move to exactly the coordinates it receives in the current master (absolute)
(2) move at the same dx/dy at which it was moved in the current master (relative)*
Similar in the Metrics/Kerning Window behavior, when adjusting sidebearings or kerning in one master, adjust them in the other masters too. There,
(1) change to exactly the same value(s) defined for the current master (absolute)
(2) change at the same difference as for the current master (relative)*
* If a node has been moved 10 down and 20 left in one master, it will be moved 10 down and 20 left in all other masters.
So, this were two additional options. Maybe three radio buttons (mutually exclusive) right in the top part of the Masters panel.
Apply adjustments to:
() current master only
() all masters absolute
() all masters relative
So it shows which master you are currently in, which masters are affected by your operations, and how.
I could even think of one more thing, to do the change only in a specific other master, so we end up with this:
Apply adjustments to:
() current master only
() [Wt1 Wd0] absolute
() [Wt1 Wd0] relative
where [Wt1 Wd0] represents a popup list from which one can select either "all" or the name of a specific master. ;-)