Ten+ years ago, when I did my externship at Le Bernardin, they didn't have a grill either. But Grilled Salmon was on the menu. So they marked off a bunch in cast-iron grill pans, quick-chilled them, and then finished them in the oven to order. If it was good enough for a 4-star NYC restaurant, you shouldn't feel bad doing it.
Now that I only cook at home, I love love love my grill pan!!!!!! I can't cook outdoors, so this is the closest I can come, and it's pretty darn great. The thing about cast iron is you can get it super-hot and get that wonderful, delicious marking. Any cooking after that is just so that the customer doesn't have to eat beautifully marked but raw meat.