Furthermore I am wondering if it is possible to overwrite the firmware to change behaviour like this. I was hoping the mobile App would help with that but it does not. This would be an awesome feature.
If you still have to the box the Fairbuds XL came in, it includes a small booklet (perhaps still hidden under the carton, together with a black bag) with some very basic instructions, mostly in picture and symbol form. Some LED meanings can be found on page 3.