the internal microphone of the XL2 is only used for applications where no absolute accuracy and precision of the microphone characteristics is required (polarity, delay, voice note recording)
Detecting feedback frequencies is one of many tasks within the set of live sound measurements, and therefore a classic task of the XL2 and a measurement microphone.
I understand that detecting feedback frequencies would also work with a non-measurement microphone. However, providing the possibility to do RTA measurements with built-in microphone comes with a high-risk of doing wrong measurements.
We therefore recommend to use the measurement microphone for all live-sound applications.
Gregor / NTi Audio
Currently there is no access to the Internal microphone from the RTA window, the reason this could be useful is to use the RTA to display feedback frequencies without using the external mic or a line input. can this be implemented? of course this would not mean you can use the internal mic as an SPL meter or to measure acoustics .