I also would like to say, sorry to hear about your loss, especially during this special time in your lives

I have seen notes in programs as well as roses placed near the couple during the ceremony. I have also seen people have a nice framed photo of their loved one on display as you enter the area. Maybe your FH and FMIL could dance to their old wedding song as their mother/son dance. I would definately check with your FH and your FMIL before you make any final decisions. It is going to be very hard for them. Bet of luck to you and your family in this tough time.