I guess I am in the minority here but I don't think it is respectful to wear whatever to calling hours. I think that you still wear business type attire. T-shirt and jeans are not appropriate.
When my grandmother passed I was appalled at what people wore. I think that we, as a society, have lost the ability to dress up.
This may not be someone that you were particularly close to but someone was and while they may not pay close attention to what you where they will notice if it is not right.
That being said, I am sure that you will put something together that is appropriate. Just based on the fact that you are thinking about it.