I haven't specifically tried this process, but some related info. I know there is a problem with the update command when using a form view. Sometimes, inexplicably the update command just stops working. I created a formview with an update command, the same as 20 other pages in my application. When clicking the update button there are no errors or application failures, but the database was not updated. It seems the update command fired properly, but for some reason the new data was not sent to the database. I have had this problem occasionally and I have found others that do too. The only way that I know to fix this problem is to delete the datasource and add it back. Sometimes I have also had to delete the formview itself and add a new one back. I know this is strange, but I have found no reason when or why this happens. Check out the ASP.NET Forums too for some more info.
Like I said, I haven't tried the postback url command with the update. The postback url may fire before the update command. Although that's not logical, Microsoft has a way of not being logical. But if you are experiencing the mysterious no update data problem, give that a try.