Your solution is better than my suggestion, but has two minor bugs:

1) eol_convention must be initialized somewhere. For instance, opening a
new editor window (Ctrl+N) and saving it fails because
self.eol_convention is not defined. I agree that is shouldn't be a class
attribute, but it must be intialized in __init__.

2) You meant chars = self.encode(text), right? (otherwise the eol change
is discraded...)