How can I touch up the paint on crown molding where I inadvertently applied wall paint?
Yes you can just take a small brush, pour a bit of paint in a plastic cup and go along and touch it up. Make sure to wipe off the brush very well, so only a little paint is applied and you reduce the chance of drips. 

Another positive you have going for you is this is up high so people aren't going to be looking at it too closely.