I have been experimenting with converting files for Kindle starting with .html files and converting them to .mobi using Calibre.

I have found that, for the Kindle at least,

<div style="text-align: center"> </div> works fine to centre text. "text-align: right" will right justify OK as well.

Embedding the style data inside a <p> </p> or a <hx> </hx> tag works as well.

I don't know about other eBook formats but I understand that all the common eBook formats are in fact based on html.