Back door as in small boat from Tawau or semporna across to Sitangkai,then up through sulu and jolo to Zamboanga,you can exit the same way.If you were desperate to get into pinas and there was no other solution its not so difficult to make connections and arrange a ride,just hang around some of the pinoys in semporna,lots of them are in malaysia illegally having been smuggled over to work in the logging camps on Borneo.