Here's an initial stab at a simple patch that just addresses the issue of 'append' not being in the mode string.  Amazingly this did not break a single existing test, though I added a new one to test how FileIO objects display their mode string after being initialized (normalizes modes like 'r+b' to 'rb+', etc.)  Most of what it tests is the existing behavior, but it also tests for the reintroduction of the 'a' mode.

This doesn't address the BufferedWriter issue, nor does it address cross-platform issues.