Your Sea Witches background, I think, is well circumscribed, though I'd recommend saying that they can sometimes be found in those locations, rather than asserting merely that they can be found there. As much as "weasel words" aren't advisable in most writing, when dealing with the touchy subject of extrapolations on lore that might affect others, they're quite handy. I would personally go so far as to say that the only area they've been confirmed to exist is the place where your character is from, which should reasonably be a fully player-created location that doesn't exist in game. That way, there's no way you can step on others' stories and you give them the opportunity (with your consent, of course) to riff off your RP to expand their backgrounds.
I would be very careful going down the road of introducing these sorts of player-created stories, songs, and the like. This gets to another issue, which is IC assertions versus OOC assertions. If your character says, "this is a traditional sea shanty of my people," that's okay. It doesn't make any statements about the world that can't be refuted; after all, your character could be wrong. If you OOCly say something is a traditional sea shanty of the Roegadyn, then you're making a lore assertion that could very easily step on toes. Note that just because there's no obvious effect on others' stories doesn't make it any less an exertion of control on them. I realize this is a fine line, but I feel it's an important one to draw in a setting where the lore is so open. As long as you're not asserting OOC that your character's statements about how traditional something is are true, then you should be in good shape.
Really, this concern about OOC versus IC assertions is really just something to keep in mind while writing this non-canonical lore (and I sure hope you'll write it up for the RPC wiki, once it opens
). Your posts so far lead me to believe that you're not actually trying to define the overall lore, just your character. Just be careful about how you phrase things and you should be okay. 
I would be very careful going down the road of introducing these sorts of player-created stories, songs, and the like. This gets to another issue, which is IC assertions versus OOC assertions. If your character says, "this is a traditional sea shanty of my people," that's okay. It doesn't make any statements about the world that can't be refuted; after all, your character could be wrong. If you OOCly say something is a traditional sea shanty of the Roegadyn, then you're making a lore assertion that could very easily step on toes. Note that just because there's no obvious effect on others' stories doesn't make it any less an exertion of control on them. I realize this is a fine line, but I feel it's an important one to draw in a setting where the lore is so open. As long as you're not asserting OOC that your character's statements about how traditional something is are true, then you should be in good shape.
Really, this concern about OOC versus IC assertions is really just something to keep in mind while writing this non-canonical lore (and I sure hope you'll write it up for the RPC wiki, once it opens
). Your posts so far lead me to believe that you're not actually trying to define the overall lore, just your character. Just be careful about how you phrase things and you should be okay.
The Freelance Wizard
Quality RP at low, low prices!
((about me | about L'yhta Mahre | L'yhta's desk | about Mysterium, the Ivory Tower: a heavy RP society of mages))
Quality RP at low, low prices!
((about me | about L'yhta Mahre | L'yhta's desk | about Mysterium, the Ivory Tower: a heavy RP society of mages))
