Hi,
thanks for your interest.
The latest tagged version can be installed on Spring dmServer. If you take the trunk you can build a version and deploy to a simple Apache Tomcat instead.
The UI part from the released version is based on Adobe Flex. Unfortunately Adobe moved Flex to the Apache Community and we are now looking for a proper replacement. A UI technology to build rich and interactive web frontends with less dev. effort. A first prototype with Primefaces did not match all of our requirements and we are currently very impressed by the Vaadin Framework.
Now back to the second part of your request: Customization. The best way to customize OpenWMS.org is to write own modules. This means UI modules (Flex/Vaadin) as well as service modules (implemented with Spring). Customization of the domain model can done using inheritance. You do not need to split your new module into several jars and keep all different layers in a single jar file. Because the existing codebase is split into many small modules (services/repositories/integration stuff) it is quite simple to exchange default implementations - whether you use OSGi or not :-)
/Heiko