Just make every character that enters combat has AP dependant on sequence.
So with max possible 31 sequence (with 10 PE, Kamikaze and x3 Earlier Sequence) player will have full AP (but not more than he had before the combat had started), and with least possible 2 sequence (with 1 PE) he'll have 0% of his AP at start.
Example #1:
1 PE, 2 sequence.
10 AG, 10 AP.
Has BRoF. (i know he can't with 1 PE, just.. example, not neccessary really)
He shoots you in the eyes with shotgun, consuming 6 AP.
He has least possible sequence, so he'll start combat with 0 AP.
But he shot you, now he has 0-6=-6 AP, he's standing with 6 negative APs, so you have chance to escape or shoot back 2 times, because he's too slow with sequence.
Example #2:
10 PE, kamikaze and max out seq perks, 31 sequence.
10 AG, 10 AP.
Has BRoF.
He again shoots you for 6 AP. He has max possible sequence. so he'll have full 10 AP - 6 for shot = 4 AP. It'll work like it's now works for everyone else.