Ah, you think someone would do this to brick a device they don't own in the field?

Then I would make the method accessible, but behind some kind of authentication method. User mails you and asks for the way to install it, once you've verified it's really them you provide the unique backdoor to that machine. Might be one way to approach it.

You can burn a unique serial number or whatever directly into the IMX6, there's a eFuse/OTP driver that lets you access it from the filesystem.

I don't think they would sabotage a device in the field, but they would check my compliance with the LGPL in hopes they could hang me on it. Maybe, I've really no idea. Would hate to find out.

Great tip on the eFuse and an authentication method! Have you ever released a product built with Qt under the LGPL?

I'd need to catch up with what happened in the BMW i3 case to understand what your obligations are to people that don't own the actual target device. At minimum, you provide a link to a repository you control with the vanilla Qt source. That should satisfy the requirement, right?

Installation of a modified library would seem (to me at least) to be a method you can share only with authorized owners.

I've released Qt products under LGPL, but they're all industrial equipment and I've never had a customer give any of this a second glance. They just want their shit to work.