What a wonderful project you have going there!
According to this article
http://ohiodnr.com/tabid/5106/Default.aspx you can transplant them after a year or two. But I see no reason not to keep them in pots longer to make sure they are big enough to make it on their own out in the yard as long as they are doing well and they are not getting root bound being in a pot. I'd check one every year to make sure the roots are not growing round and round inside the pot choking them off. I think a lot would depend on how fast they are growing for you. since the pot is 9" tall I wouldn't think you'd have to worry about the roots being too crowded until the seedlings were 9" tall too after that I'd check on them to make sure the roots were ok.
Please stop back in the forum here and let us know how they are doing!
Best wishes to you that they do well!
I'll check around and let you know if I find out anything else from anyone who might of tried this and how it turned out for them.