Lee, from the BizTalk product team at Microsoft, literally warns you against fiddling with the BizTalk MessageBox. :)
Pun apart, he gives you some DOs and DON'Ts about the MessageBox and SQL Server.
Check it out here...
http://blogs.msdn.com/biztalk_core_engine/archive/2007/01/04/what-you-can-and-can-t-do-with-the-messagebox-database-server.aspx
Regards
Benny Mathew
Seamless Integration Solutions
BizTalk Consulting, Development, Training
http://www.seamless.in