some reactions and options ---
There was in incident in 1896 involving a Japanese ship the Spanish requisitioned in Manila.
-- Interesting potential PoD.
Another additional thing that should be mentioned is: how to prevent the Triple Intervention from occurring. The key to that is to somehow stop the French, Germans and Russians from ganging up on Japan.
There might not be any triple intervention in favor of Spain in the years 1896 and 1897. And, Tokyo might even have reason to think they could get away with fighting Spain without their intervention. Because the Russo-German-French coalition diplomatic was not based on all three powers being equally interested in the Far East or all three powers being innately hostile to Japanese expansion. Rather, the Russians were opposed to the Japanese gain of Port Arthur, and both Germany and France were seeking to gain favor with Russia. This all put Russia in a strong diplomatic position. The way it worked was Germany decided that it could be beneficial to cooperate with Russia on Far Eastern matters to direct their attention east, and possibly to create follow-on opportunities for themselves. Once the Germans were supporting the Russians, the French had to support the Russians to keep the new and untested Franco-Russian alliance credible.
However, in a conflict in the Philippines and Micronesia, Russia doesn't really care. France probably doesn't care because it was probably satiated with Indochina and was reluctant to have colonial matters distract from strength in Europe. Germany may not care either, especially as long as they are getting the Shandong concessions. Even if the Germans do cry foul, they would probably find out how little Russia or France cared to back them up in the Far East. Power projection as a solo venture would be difficult for Germany and a circumstance that Japan might think it could handle.
To answer the original post (and make some variants of it), I have a few ideas:
A) Japan gets a European ally early. It joins France's side in its war on China in 1884-1885. See this:
http://en.wikipedia.org/wiki/Franco-Chinese_War
The Japanese get on the winning French side in the mid-1880s and get increased influence over a now nominally independent Korea.
When France also reaches out to Russia in the 1890s, Paris helps broker (and bribe relevant officials) for a Russo-Japanese compromise also. From that point on, Japan might feel secure enough to attack the Spanish in the Philippines if additional opportunity or provocation beckons.
So, while the 1896 incident might not happen precisely as in OTL, something could happen between the late 1880s through 1898, or if the Spanish-American war is butterflied away, something could happen in the early 20th century, leading to a Japanese attempt to beat the Spanish in the Philippines. Then of course, the interesting question becomes whether Japan succeeds, or if it has to back down or disgorge gains in the face of a Triple Intervention of Britain, Germany and America (or any two of the three).
B) Blaine beats Cleveland in 1892. The Blaine Administration is more sympathetic to the American expat run Hawaiian republic from 1893 on. Sometime between 1892 and 1896 a Hawaiian annexation treaty or strong protectorate is passed. Japan takes this as another humiliation, and feels pressure to exploit the incident with Spain in 1896 to beat a weak, unaligned opponent, in order to keep up with the Joneses. It probably gets away with it, and America in particular, gripped by an election and the Panic of 1893, will not have motive to oppose Japan in the western Pacific. The Democrats are likely favorites in 1896, and with their ideology and devotion to Louisiana sugar interests, they are less inclined to desire any Spanish sugar growing territory.
C) No Maine incident or Spanish-American War in 1898. However, other pressures still incline the McKinley administration to support Hawaiian annexation (the recent book "Pacific Gibraltar"
http://www.amazon.com/Pacific-Gibra...=1367810433&sr=8-1&keywords=pacific+gibraltar
argues that the impetus for Hawaiian annexation, while time-coincident with the Spanish-American war, had independent causes and would have been likely even absent that war)
The annexation of Hawaii in 1898 or 99 does irritate the Japanese, whose interests there had grown over the previous decade. The humiliation of the Triple Intervention is now supplemented by the German acquisition of Tsingtao as well as the Hawaiian annexation. Japanese interest in the Spanish Pacific as a sphere of destiny. McKinley and TR, acknowledging recent history, are somewhat reluctant to offend Japan over western Pacific interests. Meanwhile, Japanese confidence in its own prestige rises as a result of its intervention in the Boxer rebellion and the good press the Japanese expedition receives. However, it also increases their anxieties about the fate of the Asia-Pacific. After 1902, Japan secures its position as Britain's ally. Britain intends this to be primarily aimed at Russia, but possibly Japan exploits its diplomatic coverage to tackle the Spanish in the Philippines anyway. Events within the Spanish empire in the Philippines or Caribbean may increase Tokyo's anxieties about losing out to other powers in the Philippines and Central Pacific, especially if the Cuban conflict drags on and US intervention begins to appear likely.