I think a throne room could be multi-purpose: both functional and pretentious, as the need requires. Anyway, it's more or less a variant of an audience chamber of some sort. Maybe even the name of the room can change according to purpose.
Herald: "Cringing underling, you are ordered to appear before His Highness in the Throne Room".
Other person: "Oh, crap".
-versus-
Baron: "Let's chat about it in the audience chamber."
Other person: "Hey, don't you mean the throne room?"
Baron: "Whatever."
I was mostly offering you options to consider. In ceremonial contexts trappings and words are important. A High Seat may look like a throne and be used just as a throne is, but it isn't a throne. An office desk may symbolize power and position just as much as a throne would. Or not.
Ultimately, I think this is one of the features of the OTU that I would leave subject to referee fiat. Not in the "every ref decides for his own TU" sense, but in the "they do it one way in one duchy and another way in another duchy" sense (And in the "the father did it one way, but the son does it another" sense too).
One factor that would influence construction is whether your baron is a high baron or an honor baron (if he's a rank baron, he wouldn't have a baronial estate (though the job that gave him his rank might come with a residence)). A high noble would need more facilities and a bigger and more diversified staff for his diplomatic functions than an honor noble for estate management.
Hans