LB – Can you explain your question? “What’s the difference between the Wolf’s question and a woman asking if she can wear a shell under a sleeveless dress (that besides lacking sleeves is otherwise tznius)?”
No one suggests to wear the sleeveless shell alone, so I don’t get what you’re asking?
Wolf, in answer to your question, I do not think there is a “din” regarding this. Some extremely cautious people who are suspicious of everything they do in life might not want to use recipes with treif ingredients in them and convert them to kosher, but the average Yid would probably not have an issue with this. I just made a wonderful beef/broccoli/vegetable dish this past erev Shabbos that originally called for bison steak in the recipe.
I mean, really, if a recipe is by a goy and they call for beef, are you obligated to use treif meat in that recipe? I doubt it, so I just extrapolate and use whatever kosher meat or chicken I feel is appropriate in a recipe. I do, however, have a hard time looking at various seafood recipes and pictures showing that food, but I think that is just a personal thing.
Bon Appetite!!!