Malte,
I recommend using subsites to groups related packages. It makes good sense, and seems much better than creating a special mapping for each pair of package types. Subsites are a generic way to group any package without adding any new code. I think that using the tabs or other subsite-based navigation and the related-items package is more flexible and less error prone than trying to predict every possible combination of useful packages.
To add functionality you need to add a callback interface in project manager, for example, and then implement that callback in your custom function package. That seems like the best way to go about it with the existing toolkit. Much better than having special project manager specific code that replicates callback type functionality.
