Kevin,
I've read more carefully your messages and investigated some more.
It seems that there are several issues:
1/ To take input from a defined tty without interfering with standard file descriptors
2/ To have the result (object) of evaluation printed to a defined tty without interfering with standard file descriptors
3/ (optionally) To direct to the tty (or not) the output that is a side effect of the evaluation
Provided that no one messes with PyOS_ReadlineFunctionPointer (as with "import readline") it should be possible to solve 1 without modifying Python, and "approximately" solve 2 (with 3 implied out of necessity).
On the other hand, modifying Python as you suggest could solve 1, but issues 2 and 3 would still remain and probably require some other modifications.