I managed to get it working, at least for NPCs seems the issue is to do with draw states and their resolution order, I'd need it to be on the player obviously but I guess ill just have to pass each texture to the shader.I'm not really good enough yet this, but it's possible to directly pass shaders without going through Terraria first
Take a look at Spirit's Repo on Github, it has a decent example of how to use a shader with custom params on primitive drawing
I would also suggest Starlight River too, but I don't think Scalie wants me sharing that one around, you can find links thou in TML's discord if you wanna dig, but I'm not posting a direct one on the forums
I moved from using a terraria based shader and purely XNA so its just an Effect that I apply between draw calls.
Strangely enough the player doesn't even have a pre/post draw its just funnelled through main's draw so I'd need to utilize something like a projectile instead of custom drawing for what I am trying to do.