I may have found the law that you need, but you will still have to go to the courthouse to have it checked out to be sure... Go to this website:
http://www.legis.state.la.us/lss/search.htmIn the box next to "Law Body" type in; [color:"red"] RS [/color]
In the box next to "Title, Article, etc." Type: [color:"red"] 9 [/color]
And in the box nex to "Section #" type: [color:"red"] 201[/color]
Then is will bring up a new window, at the top of that window you will see a "Next Section" you can use this to look through articles 201 - 205 which seems to apply to marriage officiants.
If the court says that won't work, then see if you can find a sympathetic officiant thru the phone book or local church, that wouldn't mind letting your Step-Dad to be say the words, and then the official would just sign the piece of paper. Also, try to talk to a woman at the courthouse if you can, they are WAY more sympathetic about marriage stuff than men. (Sorry guys!)