Schematic for just the bass half of a fender Bassman?

I can't find one and I can't quite wrap my head around just taking a schem and not doing the second half.

Take the end of the 270k mix resistor that is connected to the bright channel volume pot wiper and connect it to ground. Done!
I would suggest 470k for both resistors instead of 270k, but its up to you.
If you want to cascade the now unused half of the 12aX7 into the normal channel, look at a jcm800 scheme. It's all the same thing more or less.

Could I use the extra stage on a switch that I could mount on the front?

Yeah, you could do something like this.
Again, it's based on a Marshall, but very similar to what you have to work with.

Thanks!