Arbitrarily sampling a texture from the custom node?

Hey everyone. I’ve been trying to write a volumetric raycaster in UE4, but I’ve hit a roadblock. When using the Custom node to write HLSL, I’ve found that none of the sampling functions (tex2D, tex2Dlod, etc.) work; interpreting the Custom Texture Input as a Texture2D rather than a sample2D.

Has anyone had any luck sampling textures from within the Custom node?

I have a little more info at my answerhub page.