Is there a tag or attribute in the BBeB specification that contols the "style" of the right margin? Specifically justified vs. ragged?

In a word no. Or not that I've found, so far.

The BBeB viewer on the Reader is set up for full justification for all non-japanese books. There is a possibility to disable justification, but it's not exactly configurable. To do that, add the following line to the <environment> section of kconfig.xml:
<variable name="bbebJustification" value="false"/>

add the following line to the <environment> section of kconfig.xml:
<variable name="bbebJustification" value="false"/>

But that would turn if off for all books all the time - right?

I wonder if you could add that to the LRF for a particular book?

The most you can do is set justification to right or center. Left/full can be toggles only globally.
BBeB was originally developed for Japanese market and many details irrelevant for Japanese seem to have been overlooked. Sometimes I think it's pretty remarkable it works well enough for Western books...