Never. End of story.

Not on the boards.

Not on the blogs.

It's not allowed.

It's against the terms of use:
6. Submissions. Lucas's company policy does not allow it to accept or consider creative ideas, suggestions, or materials other than those it has specifically requested. Accordingly, we must request that no visitors to this Site submit or send any original creative materials, including but not limited to submissions of scripts, story lines, fan fiction,characters, drawings, information, suggestions, ideas or concepts. If at our request you send certain specific submissions (e.g., postings to chats, surveys, message boards, contests, or similar items) or, despite our request that you not send us any other creative materials, you send us creative suggestions, ideas, notes, drawings, concepts, or other information (collectively the "Submissions") shall be deemed and shall remain the property of Lucas in perpetuity.

Don't do it. Don't even THINK about doing it unless it's OFF THIS SITE ENTIRELY.

Period.