Rolemaster? You are a brave brave man.
I GMed Rolemaster for 19 years (1990-2008) - it's the biggest component of my GMing experience. Two campaigns (90-97, 98-08). I learned a lot about GMing, a lot about the relationship between mechanics and playstyle, and FAR TOO MUCH about the Rolemaster lookup tables!
I love Rolemaster PC building. I enjoy some of, but not all of, Rolemaster action resolution (on the whole it gets better at mid-to-high levels, although still has issues, and some new issues arise at those levels). I really don't like Rolemaster encounter design.
We had a plot in our old game around the Lady of Pain's secret, but it never seemed to hinder the players' freedom or creativity. It started as flavor I used as DM, then the players started actively wondering about it more, then around 8th level I started to crystallize a clearer sense of what "The Truth" about the Lady might be (in our campaign). And it wasn't until the end of the campaign at 14th/15th level that I made a final decision and revealed the specifics to the players.
One sentence summary: The Lady of Pain came to be when the first Sensate took on the "greatest pain" from her beloved, a godling demon prince, and was forever transformed by it.
A lot of it was based on the PCs' attitudes toward the Sensate philosophy and inter-faction conflict. Some was based on innocent DM eavesdropping to the players. And a little was based on a 'proof of concept' adventure I ran in which the PCs got to make a decision that actually determined part of the Lady's nature (it was a "the question is the answer" situation).
Not sure if that's "meta-plot" or not...I've never had a good grasp of what that means.
What you describe here is more like the opposite of metaplot - there's a significant but ill-defined background element, and you (as GM) fill in the details, a bit here and a bit there, some of it revealed to the players/PCs, some of it purely in the background being used to shape future reveals. (From your description, I'm not sure who had control over the timing of the final reveal - you or the players.)
Metaplot, at least as I understand it, is canonical backstory that is known to the GM, gradually drip fed to the players, and used by the GM to organise the campaign and its scenarios around. So the PCs find themselves in scenario XYZ, and this ends up contributing some minor thing or other to the Great Modron March. (Or whatever it might be.)
Speakign more loosely, I think "metaplot" whenever the PCs end up being the pawns of some scheme that makes sense only to the GM in light of his/her big picture of the campaign - so that the
real meaning of the events in the game come not from what the players contributed, but from this "secret" meaning that they have in light of what only the GM knows (at least at first, until the GM does the "big reveal" to the players).
This sort of GM-driven "big reveal" play is at odds with narrativism, because it puts the meaning of things in the GM's hands rather than the players' hands. Conversely, keeping setting, NPC motivations, etc flexibile, and working them out in tandem with the players actuall playing the game, and in response to the choices of the players as a vehicle for building on those choices, I think
reinforces the players' contribution to meaning in the game. The Burning Wheel Adventure Builder talks about the campaign setting, built up in this way, as being a sort-of "history" of the campaign as actually played.