Not sure if this fulfills your requirement, but as I know, navigation portlet has configuration to hide the parent.
Also checkout if these properties help:
1 # Set the name of the display style which will be used by default.
2 #
3 navigation.display.style.default=relative-with-breadcrumb
4
5 #
6 # Specify the options that will be provided to the user in the edit
7 # configuration mode of the portlet.
8 #
9 navigation.display.style.options=relative-with-breadcrumb,from-level-2-with-title,from-level-1-with-title,from-level-1,from-level-1-to-all-sublevels,from-level-0
10
11 #
12 # Define each mode with 4 comma delimited strings that represent the form:
13 # headerType, rootLayoutType, rootLayoutLevel, includedLayouts, and
14 # nestedChildren.
15 #
16 navigation.display.style[relative-with-breadcrumb]=breadcrumb,relative,0,auto,true
17 navigation.display.style[from-level-2-with-title]=root-layout,absolute,2,auto,true
18 navigation.display.style[from-level-1-with-title]=root-layout,absolute,1,auto,true
19 navigation.display.style[from-level-1]=none,absolute,1,auto,true
20 navigation.display.style[from-level-1-to-all-sublevels]=none,absolute,1,all,true
21 navigation.display.style[from-level-0]=none,absolute,0,auto,true
Firmi prego dentro per inbandierare questo come inadeguato.