Interesting idea. You could stop the ECU from interfering by setting the correction retard and advance values to zero for FLKC, IAM, maybe even FBKC. I am a little suspicious of what side-effects there might be (other than disabling knock control, obviously!). However nothing springs to mind.
I have a small sense of deja vu right now, like this might have been discussed in another thread in the past... could be wrong though.
Anyway, I really like this idea. Consider this scenario:
1) Disable IAM by setting the retard value to 0.
2) Set FLKC retard to -0.5, set advance to 0.
(Leave FBKC as-is)
(I am assuming the ECU would not freak out and do something weird - or even just clear the FLKC table - when it sees a FLKC table full of negative numbers, decides to drop IAM, and then can't.)
3) Set up a map with a couple degrees more timing than you have proven to be safe
4) Fill the FLKC table with -2 values
5) Do a bunch of pulls
6) Check the FLKC table to see if the ECU has pulled any timing.
7) If it hasn't pulled timing, fill the table with -1.5 and go back to step 5.

If it has pulled timing somewhere, stop trying to add timing there, but still go back to step 5 and see if you can keep adding timing elsewhere.
When done, add timing to the ROM as per what you just found the engine will tolerate. Maybe repeat the process again, but this time just pull 1 degree from the new timing table, and add timing in increments of 0.25.