No answer ever came, so I reply to myself.
For upcoming LAMS release, we are using pretty much the same version of JBoss as 3 years ago, JBoss 5.1.0.
This means that no new tools are available in JBoss to solve cache access problem.
After extensive search, no solution to expose same Cache instance both to Hibernate and CacheManager using JMX/JNDI was found.
This may be possible after upgrade to newer major JBoss version, but for LAMS 2.4 another solution was needed.
CacheManager gets now Hibernate SessionFactory injected by Spring. This way it can in limited manner manipulate 2nd level cache.
This code snippet was used to look up and clear cache:
http://java.dzone.com/tips/clearing-hibernate-second-leve
Very frustrating, but this was the best we could do with given tools.