Check System

We have used this system where all our plays are colors/objects and the QB gets a signal from the Coach/Backuup QBs and calls the play in the snap count. ex:
Blue 79, Blue 79 would tell the Lineman to run the Inside Zone Rt. Because Blue is Inside Zone Right because we will run the Inside Zone until we are Blue in the Face. So the Signaler would just touch his face to give the QB the play. The numbers don't mean anything on runs.
If we would want to run it left we would use the object word that is paired with Blue, which is Sky.
So basically we can run our entire offense from the first day of practice from the LOS or the QB can audible to any play. Personal preference is I like to move the formations, see the defenses adjustment and then signal the play in on the line.
On the passes, the color/object word tell the protection and the numbers equal route schemes.
For ex: Red 23 would be a slant with an arrow, Red = 3step Right(because it gives the Defense a case of the Red A** if we can complete the little dink passes for big yards) 2=Slant for 1st Rec, 3= Arrow for 2nd Rec. (and of course Rose would be 3 step left).

So in the situation you presented with a blitz coming, if you want you could signal in the play from the sideline and have the QB call check with me in the huddle or you could give him a simple rule to live by and let him check from the field.
For ex. 4 man Blitz weak, you may want to check to a Sprint Out away or a 3 step route and Coach him up in practice what the audible will be.