Alleyways To Torah

It's quite possible that the Rav deals with this explicitly in על התשובה, I've only been through a little bit of the ספר and it is very much likely that he picked up on every nuance in רמב"ם. When it comes to רמב"ם the Briskers don't leave any part of the לשון פז to its own devices; it's part of a halachosophical construct.
From the part I have seen, at least, it is possible to formulate some sort of answer. At the beginning of על התשובה, the Rav makes a typical Brisker חילוק, distinguishing between 2 דינים of יו"כ, namely כפרה and טהרה: the former is just כתריס בפני האף וזעם and the latter changes you as a person (i.e., just as the חטא itself changes you so too the טהרה makes you a new person [reminiscent of famous רמב"ם in תשובה]). In all of the context that Rebbe pointed too (I admit i didn't look at רמ"א though) רמב"ם mentions כפרה, and it's quite possible that קבלה להבא is only a necessary component of effecting טהרה, for כפרה it is unncessary. He himself makes the distinction between ווידוי כה"ג and ווידוי בנ"י, saying that the former is about כפרה and the latter about טהרה, but this doesn't help with תשובה ב:ח which Rebbe understands to be lackign the קבלה להבא element. Regarding this last point I'm not sure how much of a question it is, in teh version of רמב"ם at which I am looking online it seems to leave out just about everything, not just קבלה להבא, so I am not sure how much of a ראיה it is; the whole נוסח is missing (my text reads: הווידוי שנהגו בו כל ישראל "אבל חטאנו" והוא עיקר הווידוי). To me it seems like there needs to be a וכו' after אבל חטאנו and there's no way of being certain that קבלה להבא is left out (looking to our version of ווידוי to fill in the gap also may fall short of being a ראיה because who is to say that he had the same version?).
בקיצור, I think one can find an answer in על התשובה, or at least it's a place to start.