The claim here is that carnism is ok as long as you don't spend your dollars directly, but if, e.g., food is going to be thrown away it's ok to snatch it before it hits the trash can at your in-laws house. Just as long as you're not funding it directly and therefore perpetuating it.
"Oh good, we don't need any tupperware to store the meatloaf because it was all eaten up; we'll have to make more next Christmas". It still perpetuates the intentional purchase of these goods to eat them. So I don't believe this is a truly blameless position, because you are still rewarding the creation of these products in some way. It just means you won't do it alone, which I guess is an improvement over doing it every day, but that's hardly a defense.
And therefore you make the vegan position weaker. People won't be encouraged to make vegan dishes or consider their ethical choices because, hey, you'll still eat it.
At the end of the day, this is your call as to how you want to live your life, but it seems that it would be an unlikely scenario that this would truly have absolutely no effect on the purchase of these goods, and in all likelihood, it's really just that you might not have considered the full ramifications and are concentrating on a smaller aspect. Probably though, this just means you want the idea of veganism but aren't really willing to put in the effort to effect impactful change, and therefore are settling for an ersatz vegan position.
It's hard to imagine a boycott of a particular product being compatible with the consumption of said product in any other context.