Tolkien issued a licence to Saul Zaentz productions in the ?early 70s, or possibly late 60s, that covered The Hobbit, the main text of LotR and some of the appendices - the timeline is definitely in there, but fewer other things than you may think. Everything from The Silmarillion and Unfinished Tales onwards is unlicensed. Most of what we have in various "geek media" is through Zaentz. I believe (take with a pinch of salt) that this licence will expire soon-ish, as in within a decade or two, and the Tolkien estate are reportedly keen to see the back of it.
D&D had to remove references to Hobbits from their books in the 70s and replace them with generic "halflings", as they had no licence whatsoever.
Iron Crown Enterprises - I can't remember who was the licencor for the MERP RPG, but I do know that did a series of character books in the 80s and 90s that covered characters from both The Silmarillion and Unfinished Tales. Very naughty, and The Tolkien Estate eventually pulled the licence from them in quite an acrimonious court case.
The licence for the current Cubicle & RPG, The One Ring, is through Zaentz.
GW said in the 2000s that they went to the Tolkien Estate to ask to use the name of Khamul the Easterling (and nothing else) from Unfinished Tales. Apocryphally, their argument was that they would call him "Bob the Nazgul" if they didn't get the rights to the name. Quite ironic really, considering GW's stance on intellectual property, but they got it.
There's enough info on the fall of Arnor in the licensed appendices to allow GW to make a supplement, but you'll notice that it's quite basic compared to the full appendices. GW's take on the appendix material has been quite cautious - there are plenty of wars in Gondor's history that would have made amazing campaigns
Christopher Tolkien hates the movies, and while the younger member of the family are said to be more open to licensed media, I believe that the board of the Estate is made up of small-c conservative scholars. It's unlikely they would issue a licence for a wargame (as that's not really the moral at the heart of the stories), and there was reportedly some disquiet among them when the GW game appeared.
On the other hand, Oathmark needs no licence to make generic fantasy miniatures, and appears to be doing a grand job already.
Coenus - I like the Warploque dragon too - seen it at shows, and it's a beautiful model.