I guess one big question is: Why?
I mean, what about Pharasma makes her unavailable to Paladins? A Paladin isn't prevented from following a Neutral Deity as long as that deity doesn't work in Evil ways, but more importantly as long as the Paladin himself doesn't do Evil deeds. Pharasma wants her followers to take down the Undead and those who raise them, something Paladins can be rather good at.
Or in this case: A Paladin "of Pharasma" would most likely be a prominent Undead Hunter, out to deal with Necromancy and protect those who risk being harmed by it. They hunt down Undead and Necromancers whenever they find out about them, they give worship on days important to Pharasma's church, they help mothers give birth when they run upon them and they oversee funerals, and all in all they uphold the things important to Pharasma. Other than that, in regards to how much they follow different parts of Pharasma's tenets it's really all up in the air as long as they stay true to the core (Kill the Undead!) and stay Paladin-y.
I guess what I'm asking is: What is it you want to do with it that separates it from an ordinary Paladin or Anti-Paladin?