Feature request: external fds on checkpoint/restore

Hello. My project has a requirement that we want to pass a FD to a container process and then checkpoint it. On checkpoint/restore the FD cannot be dumped because it is outside the container's mount namespace, so we need to specify it as an "external" FD in CRIU and pass the FD back on restore.
In CRIU this is achieved by specifying --external option on dump and --inherit-fd on restore. However, these options are missing from runc checkpoint and runc restore.
I found that there was already code in runc for dealing with criu --external options, but these options were not exposed to the user. I tried to implement them but it seems like my code has no effect at all. The patch is at https://github.com/vincent-163/runc/commit/36ee61464ff4e138aea46ffe8651569471fe9349.
Could someone please at least point me at the right direction so I know how I would be able to implement that by myself? Thanks in advance!

Start by reviewing the linked commit and the runc checkpoint and restore command entry points. Trace how CRIU options are currently assembled, then verify that external FD and inherited FD arguments reach CRIU; done means both commands expose the requested options and checkpoint/restore uses them successfully.