Slartibartfast, I think this is just yet more of a reason that you and your DH should both train yourself to refuse to give your MIL an answer, to anything, until you have talked to each other.
Seriously, get so that it's automatic to say, "I need to discuss that with [Slarti/DH] -- I'll get back to you [today/soon/next week/whatever]." Even if it's something small. Even if it seems like total overkill to check in with each other.
Your MIL has shown that she can't be trusted to share the whole story when the whole story doesn't get her what she wants. You and your DH have shown each other that you have very different priorities at times, and that it can backfire in a huge way to not be on the same page when your MIL is involved.
From experiences with my own husband, there is a limit to what I can expect him to remember or extrapolate about what my priorities and expectations are. If we have a situation like yours, where I'm not okay with another cake and he doesn't really care or think it's a big deal, I can talk to him after the fact and get him to admit that yes, he should have talked to me. But if I ask him to use the cake situation as a lesson on my priorities for anything except cake situations in the future, he's not going to be able to do it. Even if he says he will, he's not me, and he can't read my mind.
I think you may find the same thing with your DH. It's really hard to step out of your own head and see someone else's point of view, and it's even harder when the conflict is that your DH doesn't think something is a big deal and you do. When it's not a big deal to him, he's less likely to make the mental connection that he should talk to you about it.
So I think that, in the end, a better solution will be to just collectively withhold permission for anything from your MIL until you have a chance to talk to each other. Unless it's an emergency, she shouldn't be getting any answers that are not coming from a joint decision, and explicitly stated as a joint decision. It will save you having to remember to talk to each other when she goes to both of you for answers, and it will save both of you from having to backtrack (or look like you are backtracking).