GPL requires that you make any modifications to the GPL'ed work, or any derived work, available as GPL.
GPL v3 clarifies this to say you can't take GPL'ed work, modify it and make the source available but ban someone from actually using it - by patents or code signing etc.
LGPL allows you to link the LGPL'ed code to your propriety code without releasing your own code. But you have to allow someone to swap the LGPL'ed code for another version so that they can modify it.
If your app checked for a particular LGPL library (by code signing etc) then it would not be acceptable - somebody would be unable to take the LGPL lib that they had written and change it while continuing to use it with your app. You are taking their work and not allowing them the freedom they demanded in return.
You can of course do anything you want with your app's code - you just have to let them change their work in the LGPL lib