I am looking for the Rabbit 6000 instruction set, and can only find documentation that includes up to the Rabbit 5000.

A separate instruction set manual was never written for the Rabbit 6000.  You can use the Rabbit 5000 instruction reference manual at the following link;  Instruction Reference Manual- Rabbit Family of Microprocessors.  This manual’s instruction set is also relevant to the Rabbit 6000.  In addition, see the zip file which contains the additional instructions added for the Rabbit 6000.  This file is available here .  Click on New Rabbit 6000 Instruction Reference