I'm out in the weeds here, so don't pay any attention to me. Also, DO NOT ATTEMPT WHAT THIS POST DESCRIBES! It's definitely not supported.
Obviously, at some point, the customization can start to break the ability to use the Designer, or break the whole app. It would be nice, though, to have just a little deeper ability to customize some of the high level components.
In my case, I wanted to trim the Link Group list down to only the document links and change the header to make more sense.
The only way to do that reliably was to directly modify the template string in app.bundle.js.
The caveat here is that this will break if Laserfiche provides updated files. There's also a lot that can go wrong. In this case, I only modified read-only fields.
I just wanted to throw out the kind of use case I have in mind and maybe get Laserfiche thinking about some way to provide added customization, hopefully without actually needing to do a build.