Ah I see. Well I'd say stick with traditional rendering methods to accomplish what you'd like. But if you can get it to work, good luck!

Maybe I'm still not understanding what you're doing, but it seems like you can accomplish what you want with instancing. The instance data would include:
- the position of the cube
- the a texture offset to apply to the uv coords at each vertex

You send this data once per cube, and it is used for each vertex of the cube.

Sort of a step up from instancing was what I was looking for. I was wanting to have the U/V already uploaded alongside (but distinct from) the position set and then just send a pair of numbers per render call, representing the start index for the positions and the start index for the texcoords. I dug through the vert shader documentation and what I'm wanting to do doesn't seem to be do-able in Dx9. I think OpenGL may be able to do it though. I have to read through the docs again when I come back around to this project.

Thank you everyone for your advice.