@KevinH: Since ACE accepts both epub2 and epub3 books, IMHO, it'd make sense for the plugin to also process epub2 books.
The only changes that you'd have to implement for epub2 books is:
a) changing the metadata section syntax according to
this IDPF suggestion:
Code:
<meta name="schema:accessibilitySummary" content="This publication conforms to WCAG 2.0 AA."/>
<meta name="schema:accessMode" content="textual"/>
<meta name="schema:accessModeSufficient" content="textual"/>
<meta name="schema:accessibilityFeature" content="structuralNavigation"/>
b) skipping the ARIA role attribute processing step