Following up on bug # 14880, I suggest to have a place to store a "root" or "main" or "everything" category, if one exists in a wiki. There are several uses, and many wikis already make use of their local top category at various places.
The message should be optinal, default to empty, and except for some Extensions possibly offering additional goodies when the message is not empty, only Gadgets and individal installations will likely use it.
Yet, e,g, for templates that can be copied between wikis, Extension code, and others, it is useful to have one common place to look for it.
I do not know, which product/component best to choose for this request, so you are encouraged to move it to a more appropriate one.