Lab Rat was designed to have a nice fat bottom end, which is the main reason people tend to use a blend on bass, so while it's possible to put a blend in, it's not something currently on my to-do list.
I'm probably in the minority on this, but a clean blend on the Lab Rat seems like a good feature to add, provided it doesn't add a lot of components and/or significantly change the tone.
I only dug up this zombie post because I am currently looking for a Rat alternative for bass. The clean blend is a must for me. In my use, it's not so much just to add low end. I actually want a predominate clean-ish tone with some aggressive clipping in the background.