Sure, your commentary here is 100% spot on:
If you find yourself needing a control plane for your routers’ containers, you will likely need to write it yourself.
And it is kinda pedantic, but complex/hard is different than impossible 
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MT has mentioned “serial passthrough” in some forum postings, similar to USB passthrough.
Is that something that exists, or is merely acknowledged as having made it from our wishlist to theirs? That is, are you asking for serial control to be added to the list of hardware pass-through limitations, or that I missed that they did USB, and it needs to come off the list?
Serial is same category as USB, IMO. And MT ack’ed serial recently as “TODO”. See