Ok, here is where the park ranger weighs in: as long as the mice are healthy and this isn't going to be a regular thing, it's probably going to be alright. Feeder mice are a renewable resource, being bred for, well, eating (not by people, sillies!). In my mind the moral issue is less about the mice and more about the owls. The one thing you don't want to do is cause them to become dependant on humans for food. The other thing you may want to check on is if there are any laws or environmental regulations that prohibit feeding of wildlife. Something that works better than bait, though is mimicing their calls. I had a friend in college that worked for several years doing owl censuses for the US Forest Service. That was one of the methods that she and her research partner employed. The other was a dead mouse tied to a stick and waved around. She said that both methods worked pretty well.