If you can't modify your metabase, there's another option for you that involves using skip rules. It's really easy to write a skip rule if you don't care about ANY anonymous credentials and are ok with skipping them in a migration. You'd add this at the command line:
-skip:objectName=metaProperty,absolutePath=AnonymousUserPass
If you need to move some passwords, you will have to be more specific with the absolutePath, which is a regular expression. You can also set these skip rules in configuration if you don't want to type them all out at the command line. For more information about writing skip rules, see this blog entry. I tested the skip rule out and it does solve the problem.
Sorry all I can give are workarounds. Hope this helps!
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