No one ever answered this, but I'm going to drop it here for posterity, as I came across it searching for something else.
The answer is to export the weights for each mesh via the weights manager, then gather all your meshes in 1 file & re-import the weights onto each of the meshes, after adding a weight tag with the relevant joints to each mesh.
When importing the weights, it simply checks the weight tag for a joint of the corresponding name & applies the weighting, so as long as you've dragged all the joints into the weight tag prior to import, you're all good.
Exporting & importing weights can also be used to fix skinning issues in situ - for example, you noticed a problem with the weight painting but animators have already used the character in lots of files - you can simply correct the weighting in the original file & import it into the animaton scenes.
Of course, xref chracters is another alternative solution for this, as it let's you have te character auto-update.