Humm... kind of what I was thinking. Had some feedback as follows....
There is an interesting wrinkle in the last sentence of A7.531. Originally the sentence read "A leader directing fire is treated as if he were firing." That, to me, would suggest that if one intended to use a leader for Opportunity Fire that started its Turn with a firer or stack of firers it would need to be marked so in the PFPh, losing its ? in the process. Journal 7 errata changed the sentence to read "A leader directing fire is marked with an appropriate fire counter."
But that is not how the ASLRB currently reads. What it now says - "A Leader directing fire is marked with an appropriate Fire counter." - may intend to say that a leader must have ALSO been marked "OPP FIRE" or else not be able to participate (which would be my preference), but that would require more text to be explicit. As it stands, A7.531 places no restictions other than that the leader must be in the same Location at the time of the shot.
If I am understanding you correctly, your example is the one of a leader moving during its MPh to a unit/stack that was marked as an Oppy Firer during its PFPh. While A7.531 seems clear that only Infantry (including leaders) marked with a Bounding Fire counter can use Opportunity Fire, it's not clear to me how an unmarked leader can do so.
I think you mean
A7.25 Opportunity Fire. While I agree that the accepted interpretation is weird, the RB only says what the benefits of Opportunity Fire are. It does not state whether a leader can or cannot participate in it, nor does it state that a directing leader ever needs to be so marked. It is quite possible that the clause removed from A7.531 was excised in order to keep this "opportunity" available to a moving leader.
Perhaps it helps you envision the situation by remembering that a OPP FIREr can add it's full FP to a non-OPP FIRE unit's half FP. A leader in that Location can still direct that FG, but the RB asks no penalties/requirements on the leader, whether he moved or not.
And while this gameplay may seem a bit sleazy, the situation occurs infrequently. I have rationalized it to myself by visualizing that an OPP FIRER isn't necessarily firing from the start of the turn anyway. A leader certainly could run up, half-way through the turn, screaming "over there - over there!" to the units at guard...