I am in the same situation (and I suspect a lot of people with complex domain models are), which is: how do I let the user easily navigate my domain model while writing a rule.

There is support for providing a custom form within a popup when a fact constraint needs to be specified. You can present it to the user whenever a fact constraint needs to be supplied to the rule. Since you are providing the contents of the custom form, you can represent your domain however you wish to, navigate it perhaps using a tree, or even let the user perform a mini search to get to the "MeSH category".

The custom form does not currently work with DSLs though, if you are planning on using DSL sentences. But you can write your own DSL widget that does (not too difficult, if you know GWT).

For my project, the biggest utility value of drools is the standalone editor, the Guvnor rest api and the fact that it is opensource (so that I can customize it for my very complex needs)

Thanks
G. Patel