I have created a PR for the same. The changes are using inbuilt QEP and motor driver modules of BBBlue for Ardurover. "https://github.com/ArduPilot/ardupilot/pull/7117/commits/9b0f1e820c59cc71bde38e18e233c0b822b98c31"
Things i did-
1) Commented the servo out in AP_MotorsUGV.cpp and placed motor driver module calls for only use inbuilt motor driver module.
2) Created new QEP class for QEP implementation and commented the existig wheelEncoder class.
3) Added some new libraries.
4) changes in AP_Arming files.
Things needed to be done-
1) Options in GUI for choice of implementation in between BBBlue QEP and motor driver modules usage and existing servo_outs and wheelEncoder driver usage.
Hope this will be examined soon. Waiting for validating my changes.