Hello Modesto,
I don't think it's possible to assign stereotypes to Root Nodes. As you note, while other packages in EA (including 'View' packages) are represented internally as both a package and an element (a row in both t_package and t_object), Root Nodes are not -- they have a row in t_package but not in t_object. (In the API, they are Packages with, again as you note, no valid Package.Element.)
Since the stereotype is a property of the Element / column in t_object, I don't see how you could assign a stereotype to a Root Node even if you did manage to get one to appear in a property dialog. (What you describe with the Design ribbon doesn't appear to work in 15.0, sounds like a bug that has been fixed.)
if you want to see the Root Nodes in a diagram, the only way I can think of offhand is to create a
Model View with the following query:
select Name as Root from t_package where t_package.Parent_ID = 0 order by NameIf you want them in browser order, replace the order clause with
order by t_package.TPosThis will list the names of the root nodes. You can add any other columns from t_package (and the query editor will tell you what they are), but as you'll see most of the interesting properties aren't actually in there -- they're in t_object. There is a Notes column, but since you can't edit Root Node properties I don't see how that column could ever contain anything. Unless you've scripted something in there.
A word of warning.
The query editor will encourage you to include the specially-handled columns
t_object.ea_guid as CLASSGUID t_object.Object_Type as CLASSTYPEin your query. These are intended to allow the user to right-click an entry in the list and select either Find in Browser or Find in Diagrams.
Obviously you can't join t_package with t_object in this case, that's the whole point, but you can hard-code 'Package' for the CLASSTYPE. I would have expected that to allow you to Find in Browser, but it doesn't.
It also enables the Find in Diagrams menu item, which is pointless because Root Nodes can't be placed in diagrams (duh) but more to the point that function requires a GUID and EA (15) doesn't actually sanity-check this, but appears to search for all elements whose GUID match an empty string, returning a list of all elements that appear in any diagram.

So don't bother with that bit, but if nothing else you can get a list of the Root Nodes this way. They're not individual things like elements or "real" packages, but you can at least see them.
HTH,
/Uffe