Create a mission and try modify a BIS function that is called like this (by other BIS function/display onLoad handler):
call uiNamespace getVariable "BIS_unbreakable_function".
"BIS_unbreakable_function" is initialized like this:
uiNamespace setVariable
[
"BIS_unbreakable_function",
compileFinal "path_to_script_file_that_is_in_BIS_pbo"
]