You should be able to do that with a bit of clever planing:
Strip everything from a scene with all the materials you need. Then either load that scene into the new one or the other way around.
I don’t know if messiah will replace materials or makes copies, but from there it should be easy with copy and past to update everything and then delete the unused materials.
Be careful with rendergroups though. I haven’t done any extensive testing, but I often had the impression that overwritten slow materials render much slower than natively replaced ones.
What I found weird is, that the materials under the rendergroup seem to need a local material too, I wasn’t able to “overwrite” surfaces with a material assigned…
That would be cool for mass-texturing and should in theory prevent the slowness…
Cheers,
Mainly choosing different sports photos since some of the existing pics are politically too controversial.