There is currently no way to use inserts in dual mode. It is not likely this feature will be added since that will complicate the routing.

Can you elaborate on your 4CM setup? You can achieve this using insert mode with a single mono insert, and that will let you use another insert. But this setup is a bit more involved than the simple dual mode pre/post setup, and requires you to place the insert in the right place on the H90 to work correctly.