My understanding is that if you claim social security in the US, rather than Manila, then you are eligible to purchase and utilize Medigap, Medicare Advantage, and Part D.
I have expat friends who do exactly this. And as ericthai points out, some Medicare Advantage plans (and some Medigap plans as well) provide for very decent levels of coverage while overseas.
A good book for Medicare knowledge and planning is – Get What’s Yours for Medicare— by Philip Moeller. The book costs about $11 on Amazon and is available on Kindle.
Ericthai also suggests finding and working with a local agent, which I think is a really good idea if you are considering coverage beyond Parts A & B.