It sounds like a large portion of what is wanted requires work on a PC side app anyway, outside the scope of the Rockbox project.
Basically, you need to be asking developers of PC-side tools (whichever's your favorite) to be adding in support to sync with Rockbox and its database, including modifying tagnavi, rather than feature requests to Rockbox itself. We can't do any syncing from our side.
Your argument that this is "basic usability" still ignores the fact that it doesn't really require action from Rockbox's side.