In order to have access to Greenmote, as opposed to Unrefined Greenmote, you have to do the Ritual of Mania and become the Duke/Duchess because the only source is the Silo. I can see the point of this restriction prior to the succession quests, so that you have to go to the silo to get it, along the path to replace Thadon, but I can't see any reason not to be able to go there after taking Syl's place, especially after Thadon dies in the Pools of Madness.

I'm writing a quest that could make use of Greenmote if it was available, so I'm looking for anything that may arise if there's another source, or if the Silo were unlocked after he dies. Since you can get unlimited quantities as the Duke/Duchess of Mania, there should be no issue that's not Thadon-related, so I'm assuming it should be OK to get it after he's dead, without breaking any quests. Can anyone see any potential problems? In particular, could it be available after becoming Duke/Duchess of Dementia, while Thadon is still alive, or would that potentially let you kill him too soon?

As far as I am aware, killing Thadon with the Greenmote can only be done as part of the quest to take his throne (Which is conveniently locked out by Sheogorath if you choose to kill Syl instead). So even if you had refined Greenmote after becoming the Duke/Duchess of Dementia, I don't think it would be possible to add it to Thadon's meal without using the console. More Importantly, I think Thadon will bugger off at that point in the game anyway, so killing him is pretty much impossible.

In short, there would probably be no problems with adding more greenmote silos after becoming a Duke or Duchess... In fact, with a bit of sneaky work, you could actually make the greenmote from your silos ineffective on Thadon by making the greenmote you draw from them a different item to the quest greenmote (You'd possibly need to come up with a reason for this though).