For anyone interested...
There is an old WebSphere table design for storing session and attribute details in Db2. However, I heard of locking problems with this, likely exacerbated by some dubious application configuration/design choices, but nonetheless...
So I thought I would try to improve on the design. My open source solution has just completed testing. It uses a new schema, and SQL modules for all data access.
The solution is engineered to minimise lock contention and avoid deadlocks, even under high load.
The solution should be usable in its current form. Longer term I was thinking of adding a caching tier and integrating with Spring SessionRepository. I also need to add some admin processes for housekeeping.