I´ve also found another plane that could have been used in a live- fly exrcise. It´s little known, but Continental Airlines 321 was also reported hijacked, and more: it allegedly squawked the "Hijacked"- Code three times:
"report, Continental 321 over South Bend en route
Cleveland to Denver, squawked hijack three times, we have made
contact with pilot and pilot has told us everything is OK. We are trying
to determine why he squawked hijack." p. 2
"Continental 321 on ground at Peoria, FBI
approaching the air craft at this time." p. 10
911myths.com/images/b/ba/Team7_Box7-WOC-Net-Transcript.pdf