I haven’t found a lot on museums accession murals yet, but still hunting. In the meantime, as you consider guidelines, an artist’s waiver of proprietary rights, written care instructions, and written list of materials to be used (including paint types, brands, and specific colors, etc.) seem in order. The San Francisco Arts Commission has a good waiver example included in their mural guidelines. Here’s an example of a mural conservation policy by the City of Santa Monica.
I’m also checking in with a few other folks and will get back to you as I find additional information.