You have probably already encountered the intended approach for this type of problem, which is described in this older thread: for stuff multiple projects need to have the same content for, one uses files on the disk for inclusion, rather than binder items.
What you are proposing would be theoretically awesome—but a bit of a nightmare in terms of implementation (and I’m just speaking of the <$include:...> tag here in that, actually mirroring content from multiple potentially simultaneously open projects would be more than just a bit of a nightmare given the architecture of the software).