I find that I’m often switching between masters to check curvature compatibility etc., and end up accidentally editing the wrong master. (Or did I? I think I did, so I hit undo on the master, but does that undo a change I actually wanted?)
It would be helpful to be able to mark a layer as “done” and not allow edits to it, at least temporarily.
Locking sounds nice to me as well. It will also be useful for kerning, since it won’t allow you to make mistakes on already kerned/spaced glyphs.
Sometimes I also find myself editing the wrong master when the masters are close in weight. Maybe it would be an idea to have a plugin to display the master name at the top of the current glyph in edit view. Maybe I can write such a thing after the Python workshop next week!
I made a reporter plugin that shows the name of the currently open master in the Edit tab: https://github.com/ohbendy/Python-scripts-for-Glyphs/blob/master/README.md
Hopefully it helps us not get confused and edit the wrong master.
That is probably the best solution. 
I anticipate that actual locking can be very annoying, because every once in a while, you will find something small that you do want to change. So it may be the better solution to ramp up the bells and whistles.
Glad you find it useful