Progress: As a hack for exploring this issue, I fixed this in the Shell window by having ParenMatch instantiate HyperParser in such a way that it parses the entirety of the current input. In ParenMatch.flash_paren_event(), I added:

from idlelib.PyShell import PyShell
if isinstance(self.editwin, PyShell):
    hp = HyperParser(self.editwin, "end-1c")
    hp.set_index("insert")
    indices = hp.get_surrounding_brackets()
else:
    <current behavior>

With this the given example works as expected in the Shell window, i.e. the entire expression is highlighted when the cursor is after the first bracket and pressing ^0.

I still need to find a less hackish way to do this which will also work for editor windows.