(So... I think I've got some my bases covered... a little.)
Of course, you are quite right about Batman being in BIG trouble if he ever pissed off Thor or Loki. It'd be like a Batman mignon served with a lovely Robin marinade and soufflé of Batgirl for dessert. Luckily for Batman Inc., he and his team exist in the world of DC, while Loki and Thor live in the Marvel universe.
Far as I know, Thor & Loki don't have symbols like Batman & the Punisher. You could make them up. Thor's mallet would serve well, & Loki has a helmet of some sort which would do.
There was a joint series put out by Marvel & DC, where a Marvel character meets a DC character for the first time. One featured Batman meeting Spider-Man & Carnage running into the Joker, all in the same ish. The issues were later collected into a graphic novel. If I had the cash, I'd pick it up as soon as I run into it.
Well, it sounded like you were suggesting I do Loki and Thor symbols. I figured they got covered in the Norse symbol collection. Thor's hammer is, like you said, the only symbol I could think of for him, but I'm not sure how to make that.