Could a dev comment on whether a plugin could be build "in principle" which blocks one (or a number of) notebook/s from being synchronized ? My question is NOT whether there is a simple, straight forward API command to do so (obviously not). My question is about "a guess" whether existing API commands could be used to such effect.
There's no API for this and I doubt it'll ever be created.
It may be possible to achieve what you're asking for by writing to the joplin database directly from a plugin, but it's not safe and may have unintended side effects.