The UI definitions are part of the application:
modifying them is the equivalent of modifying the binary on disk.
It’s not possible because now (in the interest of relocatability
of the application, and to improve the startup time) the UI
description file is “compiled” inside the binary itself; the UI
description file is only available in the Git repository, and every
change requires recompiling Nautilus.
Given that editing the UI file once installed was never a supported
action for the reasons I pointed out in this thread, this is not a
break in functionality.
What you want to achieve cannot be done with the current, or any
future, version of Nautilus; the only way to do it is to actually
modify Nautilus so that it can do what you want.