Have a good, long, think about what Somniferous posted (replace sick only- not blanks).
If you could get the overtime junkies to play along (there would still be OT available), for even one complete month, you would totally castrate management's argument about industrial action. They would truly be stuffed having to explain that those TIBAs were ALL DUE TO LACK OF STAFF, and nothing to do with industrial action. They will try to spin it, but you would really be in a commanding position to run full page ads showing overtime was being worked to replace unplanned abscence, and TIBAs and delays were 100% down to them. You could even run ads, get press etc along the way, pointing out that you were soon going to commence industrial action after "x" date, as allowed under PIA. In the mean time, what the public is seeing is the delays inherintly due to AsA lies and the bonus-driven management culture (soooooo popular atm).
Seems to me like everyone (by that I mean all differently motivated 'types')would be willing to support something like that.