Some sort of American/Filipino enclave in the south is possible with a stronger defense of Luzon. The Japanese really want Manila as a logistics hub, and will put the effort in to taking Bataan and the Harbor Islands, T/Ls where this does not happen have significant PODs earlier. It is really the last 12 months or so before PH where reversion to the original defense concepts as well as making better use of the time and facilities there or planned over the next months before PH is a POD that does not depend on the USA to make some major changes militarily or politically - stuff that can be done on local initiatives.
The problem with any sort of "conventional" US/Filipino force holding on to some sort of southern enclave is resupply. Adequate food should be obtainable locally, but medical supplies, ammunition, etc would have to be brought in the face of Japanese sea and air control. Like the USA, in reverse, the Japanese could simply let these forces "wilt on the vine" on some southern islands - they can't really do much to harry the Japanese on the occupied islands. The could serve as a base for some guerilla operations elsewhere, but again getting significant forces from islands to island would be problematic. Of course, holdouts would be a positive propaganda/morale for the US/PI and negative for the Japanese.
As I, and many posters have said, from the earliest days of WPO the PI were going to be an outpost delaying the Japanese. MacArthur, for whatever reasons, did not plan along those lines giving the Japanese a gift far beyond tactical issues like the loss of his air power etc. Anything that throws sands in the gears of the finely tuned Swiss watch that was the Japanese plan works to the benefit of the Allies.