Stemless notes & half/quarter barlines?

Stemless notes are easy enough. To make them *all* stemless, as it seems you may want, you can right click a staff and select Staff Properties then click the Stemless option. Or you can use Note Properties to make individual notes or groups of selected notes stemless.

You'll probably have to fake the barlines using ordinary lines, at least until 2.0 comes out. Unless someone else knows some better tricks.

The best way of producing unmetrical music is to use a pre-release version of MuseScore 2 available from the very bottom of the downloads page.

The new features such as join and split measure (bar) and the ability to define a staff which does not display a time signature make it far superior to 1.3 for this.

Add to this that the kinds of barlines required for plainsong are also available in 2.0 then the argument for using a pre-release version strengthens.

I have been using pre-release versions of 2.0 for over a year now to produce the weekly Gradual for the church where I am DOM without problems. It must be borne in mind, however, that you cannot publish your work on musescore.com, you may find that things suddenly stop working, or you may experience random crashes, so it is important to make sure that you are extra-vigilant about backing up work, and to keep a copy of the version of the program with which you created your score.

HTH
Michael