At the moment this isn't directly supported but you can achieve similar results using a transparent image.
- First of all you’ll need to create a transparent PNG image a few pixels wide by some arbitrary value tall, say 200 pixels
- Select the branch that you want to centre text in
- Switch to the "Insert" tab and click the "Picture" button
- Browse for the transparent PNG and click "Open"
- Look to the "Insert" ribbon, "Picture" group and use the "Size +" or "Size –" buttons to set the branch height to a suitable value for your needs
- Open the Position menu and select "Centre and Behind"
That will have the effect of vertically centring your text within the branch.