The version of Hypersonic used in EAP 4.2/4.3 is 1.8.0.

1. Hypersonic 1.8.0 raises exceptions when trying to connect using a URL containing an IPv6 address literal:
"jdbc:hsqldb:hsql://" + System.getProperty("jbosstest.server.host.url", "localhost") + ":1701"
which arises when jbosstest.server.host.url is defined and points to an IPv6 bind address.

The exception is: "java.sql.SQLException: SOO10 Invalid argument in JDBC call"

2. Hypersonic 1.9.0 has a fix for connecting to a IPv6 host. However, performing a simple replacement of jars results in a number of errors which were not previously seen with the 1.8.0 jar. For example, when starting up the AS, problems are seen with JBoss MQ and its database use:

It appears as though an upgrade to Hypersonic 1.9.0 may be required, although this will probably result in fixes to components which depend on it.
It would be simpler to patch version 1.8.0 to contain the IPv6 fix and retain Hypersonic 1.8.0, unless other features from 1.9.0 were required or there is a need to maintain up to date versions of Hypersonic to ship with the product.