RC Funeral etiquette - question about the post luncheon after the funeral?

Generally, there isn't a formal invitation to a wake (luncheon) after a funeral - it is just generally accepted that some sort of get-together will take place and those who attended the funeral are welcome to attend. If they do ask you whether you are going, perhaps just politely say that you are very sorry but you have to get to work or to school (or another equally valid excuse) and that you are very sorry for their loss. Perhaps say that you would love to catch up for coffee or something next week to see how everything is going? At least you're showing that you care and are there for them.