I am the author of the ODT reader, so I might be able to help.

The short answer to your examples is this: there is no simple markup.

OpenDocument files split structure and styling into two parts. Either into two separate parts in one stand-alone XML or into two separate XML files in the ODT zip. "Centring something" is part of the styling in this model.

So if you want to centre something in a filter, here's roughly what you need to do:

• Create your own template based on the default template
• In that template, create a custom style that looks the way you want, including centring.
• In the filter, create your elements and simply apply your custom style to them.

This gets more fun with images because images are floating elements, possibly in a frame. One of the simpler ways to style them is with the same approach via "frame styles". That doesn't sound surprising, considering the names, but frame styles are not the best-known feature of ODT.