If you pull that off it will be the coolest Burrito ever!!!
Most of the guys just used the rearend of a donor bike and make the front section. Conduit should work, I would use something a little stiffer to keep flex to a minimun since she'll be so low.
The pulley set-up would put the seat in an awesome position "super low" but it's a little tricky making sure it all lines up and works smoothly.
Go for it, if it doesn't work....so what, the seat comes up a few inches, no biggie.
I tried something really stupid once, it worked and was rideable, but it never was really "smooth" in the drivetrain department due to the pulley set-up.
I think I still hold the world's lowest seat height on a Burrito record though!
Video clip of it rollin'............