Could you cut out the bottom (1/4-1/2 inch) on a pocket door while it is still in the frame? What tools would you use? How would you make sure cut is straight?
Hire a Door Trimming Saw.
In the UK HSS stocks them.
I'm years late to the party; I just came across this question. I'd use an undercut saw (a/k/a a jamb saw). An online search will show you images, and you'll see exactly how they work. They're made specifically for cutting a narrow slice off the underside of baseboards or door jambs, plinth blocks, &c. so that a (higher) floor can run under them. They're a specialty tool so I don't know about renting. A good flooring installer will likely have one.
I just did this myself. I have a pocket door that is NOT coming out, and in order to tile under it I needed to shave about a quarter inch off the bottom. I ended up renting a Crain jam saw for like $30 to do most of the damage. It cut most of it, but in order to get the rest of the door back in the frame I used a kobalt multi use saw to finish the cut. Its a bit of a pain, but if you want to get it done you'll work through it. Didn't have to rip out any wall, and my door is cut properly.