are you using an XP-8?  That's the only way you're going to do this.

There's a "cancel macro if pending" option you can throw in there in some sort of variable. 

It'll work, just have to think about it for a minute to give a better explanation.

edit -

in the XP-8 system macro - go to the System Macro drop down.  you can select "cancel system macro if pending" from there.

So you could have a Power Off system macro, then maybe a "Don't turn off yet!" button next to it.  The entire "Don't turn it off yet!" macro would be:  "Cancel pending system macro: Power Off"