I've been able to get there (on MS SQL Server 2005), with thanks to directions at http://sqlserver-qa.net/blogs/tools/archive/2007/09/09/value-of-9-6-2007-12-00-00-am-is-not-valid-for-value-value-should-be-between-mindate-and-maxdate.aspx about how to open, save and modify existing Maintenance Plans.

It's not as straight forward as I'd like. I still have to put some effort into changing connections and due to having named instances, we have to change MsDtsSrvr.ini.xml.

It does all work and seems to be less work than creating the Maintenance Plans manually, manely due to our preferred connection settings in the Maintenance Plans. But it is a bit of a hassle that you cannot just use one product to do it all. And I do wonder if I could get this to work if I had two named instances on the same server. I'm not sure how one would change MsDtsSrvr.ini.xml for that.

If anyone would like a detailed description of how I got it working, let me know and I'll post it. Or mail me directly.