As for Sean's questions, I originally wrote the rules to admit only keyboard accompaniments because that will usually keep the focus on the voices, which is what this competition is about. But if you do submit a work with orchestral accompaniment, the judges should still look at it and judge it fairly.
Remember! The rules are very flexible! So stop asking questions - just write good music!!
As for submitting already completed works, I see nothing against this. If one thinks their older piece is just as suited to show off how they work with voices, then by all means.
But, I would strongly suggest
against submitting pieces that have already been posted on Young Composers.
Being at a music conservatory, I am positive I could find a couple of singers to lay down a piece - whether or not it was written for a smaller group, or we just do it as a quartet or something, maybe laying tracks over to get a full chorus sound. Of course, a solo would be easier to find, but smaller groups are feasible as well.
Sean - Are you going to participate as a judge or composer?
Pianoman - There is no deadline as of yet.