I think I have found out where the problem is. In fact, the encoding of the interactive input is determined by sys.stdin.encoding, but only in the case that it is a file object (see https://hg.python.org/cpython/file/d356e68de236/Parser/tokenizer.c#l890 and the implementation of tok_stdin_decode). For example, by default on my system sys.stdin has encoding cp852.

>>> u'á'
u'\xe1' # correct
>>> import sys; sys.stdin = "foo"
>>> u'á'
u'\xa0' # incorrect

Even if sys.stdin contained a file-like object with proper encoding attribute, it wouldn't work since sys.stdin has to be instance of <type 'file'>. So the question is, whether it is possible to make a file instance in Python that is also customizable so it may call my code. For the first thing, how to change the value of encoding attribute of a file object.