I am not much of a meta guy but I do understand that the meta sites are usually scoped to their related main sites.
I saw a proposal on Area51 and I liked it. Since there is a related existing SE site, I am thinking to start a meta discussion there so that the users there can follow and/or contribute the proposal, if they like.
I feel this is fine as this is just a meta post for discussion, but asking here for confirmation.
Of course, I wouldn't have promoted on a meta site if the new proposal wasn't related to it.
Note: I understand if I have to discuss about the proposal I should do that on Area51.SE itself.