scm url not being parsed correctly for release:perform only

mvn release:perform -DconnectionUrl=scm:cvs:ext:username@cvs-server.mycompany.com:/cvsroot:projectname
works, except it won't really do what we need without a lot of manual intervention, because this command:
mvn release:prepare -DconnectionUrl=scm:cvs:ext:username@cvs-server.mycompany.com:/cvsroot:projectname
yields this error:

[ERROR] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] The scm url is invalid.
  - The connection string contains too few tokens.
[INFO] ------------------------------------------------------------------------

-e doesn't reveal a long enough stack trace to show anything. It would be helpful if this error could say what url it's parsing
the scm section of the pom is this:

	<scm>
		<connection>scm:cvs:ext:cvs-server.mycompany.com:/cvsroot</connection>
		<developerConnection>scm:cvs:ext:cvs-server.mycompany.com:/cvsroot</developerConnection>
		<url>scm:cvs:ext:cvs-server.mycompany.com:/cvsroot:projectname</url>
	</scm>

We don't intend to put the user name in the pom (shared resource) which is part of why I'm trying to override it.

and the servers section of settings.xml is like so:

	<server>
	  <id>cvs-server.mycompany.com</id>
	  <filePermissions>664</filePermissions>
	  <directoryPermissions>775</directoryPermissions>
	  <username>username</username>
	  <privateKey>/home/username/.ssh/id_rsa_of_this_setup</privateKey>
    </server>

but in any case I'm not sure it's even reading the scm section of the pom correctly. After downloading the plugin sources, one of the few unresolved items was the Xpp3Reader which might have told me the answer. This is not up to my experience of a high bar set by working with this plugin in a subversion environment, but maybe it's an unfair comparison.

Any attention greatly appreciated

--Ben Clark