Łukasz Antoniak, who is a regular contributor to Envers (thanks!), recently finished working on HHH-5580, a feature which enables you to quickly retrieve the entity types that have been modified in a revision, by optionally storing them in a dedicated table.
Łukasz wrote an excellent tutorial on the subject, so if you are intersted, be sure to read it!
The feature described is available since the Hibernate 4.0.0.Beta2 release.
Adam