First, you need to be familiar with the tools were`re talking about: ROS, APM, MAVLink, ... You're not going to be able to evaluate the feasibility before and I cannot do it for you as you can understand
MAVLink is just a communication protocol, so even if you create new messages, you have to feed them somehow. In the case of the motor spin. the
STABILIZE mode provides a direct input to motors using
You can also have IMU readings, ... But I cannot state if it's gonna be enough for you purposes.
So, again, get familiar with APM, with its Ground Control Stations (such as APM Planner, QGroundStation, MavProxy or Mission Planner), MAVLink (to see which information APM sends to GCS), ... Once you gain understanding of how they operate, you'll be able to determine how your project could be done. One case could be that APM exposes enough information through the MAVLink protocol and that using
STABILIZE mode and
MAVROS you can command the drone. Or just not and you need to hack APM code for your purpose.
To have a direct conversation with APM, MAVROS and MavLink developers, the best is reaching them at Gitter. They have their own channels: