Sorted, all in all wasn't that difficult:
https://www.youtube.com/watch?v=IbbBHgS1Dm8On the DTC side, that is. One thing I do not know yet if the class that I have chosen flips the MIL on. The status of the DTC says no, but so it does for all the other DTC when I have them enabled and the ECU is on the bench without an actual MIL connected.
Following your hint I looked into the limp mode as well and decided to give up on the idea (to trigger one on the sensor fault). Trying to hijack any of the existing limp modes would result (my slightly educated guess tells me) in the other procedures immediately disabling it back, or freaking out otherwise. Implementing a whole new limp mode is beyond the purpose - with a faulty FlexFuel sensor the car would limp anyhow and the MIL on the dash would tell me to go easy on the engine anyhow.
To be continued... (I guess I should also move these ramblings about my developments into one place instead of scattering it all over different topics).