re: Unaffiliated Reception Halls in Baton Rouge?

I'm pretty sure the Louisiana State Museum, Old Governor's Mansion, & the Old State Capitol do not have their own caterers, but they may have a list of approved or preferred caterers that you have to use.

For someplace a bit less expensive, you might try some of the hotels to see if they have a banquet room available. I don't know if they would allow outside food or not.

ETA: Also try some of the BREC facilities--maybe Magnolia Mound Plantation?