If you've lifted the hood on the implementation of the Roots table, you'll see there's a defaultAtts table in there.
That's the equivalent of the attributes on the node that's being displayed. So it has a type element, that's the nodetype. Any of the other elements in that table are available the code that processes stuff.
Basically when you're editing the prefs for a root outline, you're editing the entries in this table.
There may not be a user interface for the pref you want to set, but if you have access to the database, you don't have to wait. Just edit or add the value you need in the table for the root.
So, if you need to create a river node, just drop in there and set the type to river and add a url element that's the link to the JSON for the river.