What's the write equivalent of volume sample?

I want to use findattribval to find which primitive is named “vel” for instance (but having the name of that attribute set as a parameter on the volume vop). Then I can feed the output of findattribval into volumesample to get the volume data without knowing the primitive ahead of time.

I would just do it with bindings, but a) I have to know the name of the volume ahead of time, and b) binding everything to density doesn't seem to work for vdb vector grids.

So, basically, I want to write voxel values in the same way that I am reading them when I use volume sample.

Is there a good way to do that?