Op, I get where you're coming from, but I think the way you did it was pretty rude.
At my IL I find eating quite a strain. Partially because MIL thinks everyone eats like they do or is wrong. So she thinks everything should be finished on a plate (and gives huge servings, with some of everything) and she is also inclined to make things with mashed potato which, I hesitate to say I have a phobia of, but I really will vomit if I try to eat it. (yes dh has mentioned that to her several times. Often the conversation goes along the lines of him mentioning it and her following it up with something like "don't worry, I'm doing shepherd's pie." 
) I'm also fussy and find any pressure on me to eat makes it much harder to eat anything-even stuff I like.
Dd1 used not to be fussy, but had pneumonia a number of years ago and became overnight very fussy. Dd2 was fussy and now isn't and ds will eat anything.
So I've worked out strategies to deal with it.
I start off by usually asking for a small portion. She sometimes asks if it's enough and occasionally humphs that I'll die from not eating enough, but I have got her down to reasonable portions now.
I make sure I'm next to dh, (or ds sometimes) and when she's turned I'll remove what I don't eat onto their plates. Dh will sometimes distract mil while I do it.
If it's something which I know is a major issue and I really can't cope with having I may find a reason why I (and sometimes dd1 if relevant) won't be there. Or I'll say we'll be late and don't wait for us. (eg going blackberrying is a good one as we can then make a pie when we get back). That's one to be used once maximum though.
I also help with cooking (as does dd1 and dh) and say things like "Oh do you think we could make lasagne instead of shepherd's pie. I'll do it if you like." Don't pester, but suggest once. Or maybe "what about roast potatoes for dinner. Dh makes really good ones."
I also have my afternoon wander. There's a shop down the road. I go down sometimes with dd1 and we buy a roll each and a drink and eat it if we've been really struggling. I also take a packet of nuts that I can eat in the evening.
But at the same time I talk to dd1 and say to her "you will try and eat this part of it, and I will support you not having this bit." And sometimes I'm choking down stuff I find difficult, but I do it.
I think in your case I would have, in advance, said "dd2 is really struggling with food and after a long journey won't eat much, so if it's okay with you, I'll bring some sausages and chips and cook them for him-or everyone if you prefer.
I would then have made some excuse on not being hungry and taken only veg, or whatever bit I wanted to eat. And made some compliments about what I was eating.