Use of micro-aerial vehicles for inspection, surveying, transport, and many other tasks in society has become increasingly popular. As research on these platforms develops, many researchers may wish to deploy new algorithms directly to the embedded flight controllers used by these vehicles. The PX4 Autopilot software stack is the de-facto choice for these applications. However, interfacing with this mature and highly abstracted software stack may be challenging. In this paper, we consider a quadrotor with an extended long-reach end-effector, which generates an unmodelled torque disturbance. We show how this causes degraded performance with the standard PX4 attitude controller then provide a guide for the design, development, and deployment of an Extended State Observer (ESO) directly into the PX4 firmware to improve the attitude control performance. We open source the code used for these experiments for other researchers to use in their own experiments.