What I ended up doing was just modifying include/register.php so the if-statement that runs the validator also checks to see if you're trying to validate an APO:
PHP Code:
if (!@$error && $current_area == "C" && !empty($active_modules["UPS_OnLine_Tools"]) && $s_state != "AA" && $s_state !="AE" && $s_state != "AP" && $s_city != "APO")
If any other cases pop up in which it shouldn't be run, I'll probably modify this code to reflect that.