Malte,
I am not sure what you mean. The paramter should be set by the administrator. Here is the scenerio:
I install file-storage and the file-storage-dav package. I want file-storage package instances to be WebDAV enabled by default so I set the parameter in the file-storage-dav package.
Now whenever I create a file-storage package instance I need to add the folder_id of the package instance to a table to enable the WebDAV support.
One way to do with is with an after_instantiate tcl callback. I could create the callback in file-storage, which would check for installation of the file-storage-dav package, and if it is not installed, do nothing.