Throughout this forum I have seen many posts about being able to add extended information to table columns. The common solution is just to use extended properties feature of Sql Server. There are couple of issues with that approach:
1. Only works if using Sql Server for back-end
2. "Code Generation" information is now stored in two places - template XML and Sql Server.
To counter those issues, I chose to write a custom template property which displays a GridView with table columns and allows to add custom information for each column. You can read about "ColumnGrid" custom property and download it from here:
http://www.entechsolutions.com/Blog/ViewPost.aspx?PostID=8
Best Regards,