Speaking as someone whose DH was programmed to the "reasonable one*" while his sister wreaked havock, I can only advise you to live your boundaries. If you enable him to continue the "just this one thing" cycle, you will never pull out of it. Your DH is used to appeasement, to peace at all costs. He knows he can depend on your love, so he expects you to compromise because he can't trust BIL's love. So what you have to do is make it just as uncomfortable for DH to put you in the "compromise" position as it is for DH to confront BIL.
DH can plan to "honor his commitment" to appease BIL all he wants, but you will not be involved. If he brings it up as planning for BIL's bday dinner, tell him, "I will not be here. I will be at a movie that night, but the crockpot is in the cabinet left of the stove." When he protests, tell him, "I will not be there." And when the appointed night arrives, go to the movies. He can accuse you of not being supportive or showing BIL you're afraid of him or some other nonsense to try to force you back into the position of helping him appease BIL - because that's what DH's comfortable with. He doesn't know any other way to operate. But ultimately, he needs to hear the same simple, repeated message that BIL hears, "I will not have anything to do with BIL."
*A friend of mine called it the "Curse of the capable" over the weekend and, boy does that describe the situation.