I think it should be very easy to remove the less than 0 checking and thus allow negative donations. Try editing the donate.php file and search for this:
PHP Code:
if ($amount < 0 || $amount == 0) {
eval(standard_error(fetch_error('error_icash_sendmsomthing')));
}
and place a double slash ( // )at the start of each of those lines or make a backup of the file first, and then delete those lines.
Just tested it - works fine.