My curse perk has now triggered a couple of times when subdued. Thought that this wasn't a thing, so probably bug.
Or King of Worm is just that awesome.
Can confirm this happening after I defeated someone in a duel months ago.
I think you are allowed to benefit from it triggering on the first instance of being subdued in a pvp but not again thereafter while down/already subdued.
It makes sense insofar that most hostile PvP starts on a subdued mode.
Curse perk effect does not override being subdued. If you were subdued, stay down as per the PvP Rules (http://www.efupw.com/forums/showthread.php?40-PvP-Guidelines).
If your curse perk kicks in after you've been subdued, then fate has graced you with terrible timing.