Sadly I don’t think rst offers that ability… the table styling options are rather limited. That said, if you’re rendering to HTML, you could add a custom stylesheet with a css rule such as:
table.right-align-right-col td:last-child {
text-align: right
}
and then add the directive:
.. rst-class:: right-align-right-col
right above your table in the rst file. It’s clunky, but it should work.
update 2013-2-6: I’ve since needed to accomplish this myself, and came up with a more permanent solution. The cloud_sptheme.ext.table_styling Sphinx extension adds directives for doing column alignment, per-column css classes, and number of other table styling tricks. Despite being packaged as part of the “cloud” Sphinx theme, it should work with any Sphinx theme.