It would be
Code:
local BackdropFrames = _G[AddOn.BackdropFrames[i]]
unless the table stored the reference to the created frame rather than its name.
It would depend on whether the template (common method for adding a mixin) adds any frame elements required by the mixin. If all it's doing is applying a bunch of methods to an already existing (or created in the process) frame then that would work. Or you could add the template frame elements "manually" when you apply the mixin if you weren't responsible for creating the frame(s).
p3lim's post seems to indicate, for backdrops, that it's just applying the mixin methods.
You just need to make sure you've applied the mixin before any of your code calls any of the methods on the frames.