You should be able to change the Multipass save to something like PNG or JPG which will give you all the active Multipass elements as separate files. You can disable the ones you don't want in the Corona Multipass set up, probably uncheck the default save of "Regular Image", to avoid getting extra files you just have to delete.
Of course you won't be able to adjust the lightmix in post with this, and would need to re-render the entire animation sequence to get a different LightMix result.