Hello p.schoeberlein,
So instead of exporting the model from UE4 after you have already imported it into the editor, is there a reason you are not just modifying the model in Modo and then just re-importing the mesh?
This would avoid the issue of having your material instances needing to be exported, and essentially doubling your workload. If you already have the model on your computer, and since you are not actually modifying the geometry within UE4, you can edit the model in an external program, re-import the changes, and they will be applied with the material instances.
With that said, we actually do not officially support Modo and I do not have the software on my computer to verify and test this workflow with that application. We do support and have documentation on the suggested FBX Static Mesh Pipeline using 3DS Max and Maya.
Let me know if you have further questions or need additional assistance.
Thank you,