There is a special saw made by the Crain tool company called the No.800 Super Saw. It's designed to undercut doors (pocket doors included) without removing them from the hinges (or rollers-in the case of a pocket door). It's not a cheap tool, so buying it for this one job would be very expensive, but if you could find a local craftsperson with this tool, he/she could solve your problem in minutes!!
The Crain tool company's phone number is (408) 946-6100.