UA tried serving MNL from NRT, TPE and SEL in the 90s. Perhaps that's one of the reasons it did not sustain enough business there. To build a presence, you need to have a fixed connecting point between North America and MNL.
MNL flight kept getting switched in and out of NRT to make room for additional intra-Aisa or North America seasonal or permanent services in the 90s. There was a lack of consistency in schedule and route planning.

I am certain there is enough traffic into MNL from North America, but you do need either NRT or HKG as a permanent connecting point. UA operates multiple flights into NRT and HKG to be able to feed traffic into a single MNL flight. TPE and SEL never had enough inbound or local traffic to generate enough demand to fill up a 747.

I like the HKG idea. It is closer to MNL than NRT. Also, there are significant demands between HKG and MNL and between Southeast Asia and MNL. UA could generate MNL traffic form SE Asia using its SIN-HKG flight. Add local HK demand to MNL with feeder traffic from North America, you may just have enough demand to build a business case to justify the HKG-MNL route.