As for Showing the Flag the Pre Dreadnought was much stronger then most of the smaller nations had in there fleet . And they would of been great ships for colonial navys to of used as training ships . What if in the 20's and 30's the US had of sent to the Philipines a sqs of Pre-Dreadnoughts and Armored Cruisers for the Philipine navy to use as there navy . And they were there with the US Asian Sqs of the US Navy They would of helped the Philipines fight the landings of the IJN .
For a humanitarian way of thinking, this idea would be great, although politically difficul, as the US forces would allow the secondary considered Phillipino people arms of some advanced types, especially tachnology, they previously lacked, dispite its obsolete nature. The forming of a seperate Phillippino Navy was possible, but would be lacking several key ingredients, namely the infrastructure to maintain a standing Navy. This is not only the necessary hardware, but more importantly the training and education institutes needed to have personell trained to at least the basical principals of Navigation, let alone operating a complex warship.
Another problem would be the value of the theoretical Phillippino Navy as a suplementary force to the USN in the region. The basical lack of experience for the Phillippino Navy in operating and especially lack of modern warfare was even bigger than for the equally unprepared USN Asiatic Squadron. The ships were too old and too cumbersome to be of any use in the Island rich Archipello, while very vulnerable to all sorts of attack, especially from the air. The Pre-Dreadnought and the old pre WW1 Armored Cruiser was not capable of dealing with modern warfare WW2 style adn the IJN would have had no problem in dealing with them. (The Formosa based IJN and partly IJA Airgroups were especially trained to engage shipping, so the Allies were defenitely in a serious state of problems, when war eventually would break out.
In the Invassion of the Phillipines, I see no difference, how the Japanese would have done it, compared to the OTL, as they already had the superiority in the air in the region, since the USAAF in the Phillippines was seriously understrength and equipped with inferior aircraft mainly. Only its Strategic Bomberforce was modern and even this was understrength, lacking the needed number of planes to do some serious bombing.
The theoretical Phillippino Navy, equipped with a few old Pre Dreadnought type battleships, perhaps partly modernized, besides a few old armored cruisers, would be a bit unballanced, as the needed support was lacking of smaller Naval forces, since the USN could not spare enough destroyers, due to other demands elsewhere (such as the Lend and Lease act). Perhaps only a few old flushdeckers could be given to the Phillippines, but these would be likely operating with the USN forces more coorperately, as the USN too had several DesRons of the same type in Asia. These vessels were out of date, but prooved to be the USN's most usefull tools in the early stages of the Pacific War. The Phillipino DD's propably would have done equally well, untill finally hunted down by far more superior Japanese forces, simmilar to the otehr Allies.
As for the capital ship, these were likely to be used defensively, given the lack of airsupport. Most likely, they would have been abbandoned when personel was needed on land for manning the defenses on the ground, while part of the crew would man the guns, to act as a sort of floating battery in defense of the erea, where the ship was. (In Russia, something simmilar happened with ships, closed up in ports, threatened by armies on the ground.) Most likely the ships would be lost anyway, either by running out of ammo, or by enemy (air or ground) attacks. When venturing out at sea, they would be vulnerable to attack by both IJN air and navalforces, especially by the number of IJN submarines, patrolling near the coastline.