The Tydings-McDuffie Barong Tagalog, circa 1930's

Those familiar with Philippine history will note that the grant of independence from its colonial status as a U.S. possession was enacted under the Tydings-McDuffie Act of 1934. It stipulates that Philippine independence will be granted ten years from the law's enactment. Thus, by law, Philippines will be free of U.S. control on July 4 1944 after a period of "democracy-in-training" Commonwealth self-government. Unfortunately, WWII intervened and independence would not be celebrated until 1946  after Japan was defeated and driven out of the Philippines. Lest you feel gratitude for the U.S. Congress, know that the grant of independence, though accomplished through the skillful manipulation of circumstances by Manuel L. Quezon and his fellow politicians, was not free of U.S. self-interest. The act reclassified Filipinos from U.S. national to alien and new immigration limited to a quota. In the 1930's the increasing number of competitive Filipino migrant labor in the U.S. was problematic to American labor unions and their political supporters. Tydings-McDuffie Act was followed by the Filipino Repatriation Act that provided free repatriation of Filipino farm laborers back to the Philippines. With a quota limit to return, few Filipinos took the offer. 
The Tydings-McDuffie Act nevertheless led to a heady appreciation for symbols of Filipino national identity. Quezon, a.k.a. MLQ to many, was elected the first Commonwealth President. Quezon was also a fan of piña, as were the cultural and political elite during those times. Piña dresses, called
terno (butterfly sleeves, made internationally famous later by Imelda Marcos) were the rage in U.S. Filipino and American women wore lavishly embroidered gowns for social and official occassions. To commemorate the Tydings-McDuffie Act, a piña Barong Tagalog believed to be commissioned for MLQ was embroidered with tiny Philippine and U.S. flags and embelished with tiny ribbons embroidered with lettering that spelled Tydings-McDuffie.
MLQ was known to wear the barong tagalog at political rallies and public speeches, the dress itself gaining traction as Barong Tagalog during this time. Before this appellation, it would have been a generic embroidered piña shirt called barong lalake (according to Nick Joaquin) that became popular in late 19th century, specially with the ilustrados.  It is uncertain how the label " barong tagalog" came about, but it is believed to have emerged during the 30's. I would imagine, given the linguistic regionalism of Filipinos, the Ilokanos or Cebuanos would have questioned the label, although now, it would be hard to imagine what a "barong Ilokano" or a "barong Cebuano" would be.  Several things come to mind that favored the label "barong Tagalog". One, is that the Tagalog provinces Laguna, Tayabas (pre-Quezon province name) and Batangas were known for their embroidery work which is associated with the shirt. Additionally, MLQ, was an avid piña fashionista and he also is recognized, under his presidency, as the "Father of the National Language" where Tagalog became the basis of the national language, otherwise to be known as Filipino. Literary historians have also noted that the 1930's was the golden age of Tagalog literature. Thus by association with MLQ, piña, and Tagalog embroidery and Tagalog language, it made for an easy popularization of this idiosyncretic male dress shirt as the Barong Tagalog that we know now.