No approach or departure procedure can cater for all contingencies. 2.5% is what you’re required to achieve to give you the minimum obstacle clearance of 100FT. How you achieve that is up to you regardless of how many engines available to you at the time. If you have to add extra to your minima, so be it.
As Lancer alluded to, a raging wing fire most definitely constitutes an emergency. I couldn’t imagine ANYONE trying to hang you for busting minima in order to get the aircraft on the ground in that case.