So I'm going to be doing sprite renders in Mental Ray for a while and wrote some custom shader networks in the Hypershade. On one, I have 12 or so attributes around the network that need to be set per surface/asset. What I want to do is have an easy collection of them, so I don't need to dig through the hypershade each…
It turns out that IPR acts the same as the viewport with the upstream connection in place. It only fully evaluates the network on a full render, which is still quick-ish. I did write a "switcheroo" script to swap all the extra attribute outputs from the material node into inputs, which gives a one-way ticket to each…
I wonder if you could use Shader FX instead, and capture the viewport instead of render it? Then wysiwyg. Getting alpha would be a bit trickier, but you could use a shader boolean to switch to alpha-only mode, then combine the two captures into your preferred image format.