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Guess you're aware you're running a high risk with such a thing.
Anyway, assuming you know what you're doing:

No need at all to change ANYTHING in core scripts.
In section 3 add:

for ('id', 'subject', 'body') {
    $tbb->{'post'}->{$_} = $tbb->{'get'}->{$_};
}

You'll have to do the cleanup yourself of course regarding URL decoding. But that can be done just as well in section(3) of entry scripts. ... or in a custom plugin loaded in that section.

Rainer