In the OTU[*], all the Metal Worlds belong to the Border Worlds and have a population level of 0 (at least officially) in 1117 [RgS:45]. In 1115 a group of people from Lunion had attempted to settle there illegally, but when Norris became archduke, he decided to appease the Border Worlds and allow them to expel the illegal settlers [SW:68]. In the GTU the Imperium (who wasn't distracted by the Rebellion and the (assumed) imminent threat of Zhodani invasion) decided to settle the controversy by buying all four Metal Worlds from the Border Worlds [SW:21]. The Lunionites stayed on Steel and a colony was se up on Mithril in 1119.Anyway, if the Imperium tried to develop them, I guess there would be quite a lot of diplomatic conflict, as the BW would lose the legal facade of independent, and sure this move would be diplomatically apposed by the SW and (I guess) quite a lot of people in the BW themselves.
Of course this will only add more interest to an adventure about developing them, either by Sword Worlders or by Imperials.
[*] Prior to any MGT retcons.
Hans