An preliminary block should be available at itamart/moodle-block_course_navigation. I've made it display only the sections. The configuration of the block allows you to set a hierarchy in the custom menu style
First section|sid
- Sub section|sid
-- Sub sub section|sid
Another section|sid
- etc.|sid
- etc.|sid
(the sid stands for the section id of each section in the list. Must not be removed)
The hierarchy can include any subset of the available sections in which case in subsequent editing of the block configuration the sections which are not included in the hierarchy are added below it "commented out"
First section
- Sub section
-- Sub sub section
Another section
- etc.
- etc.
# Section not in the hierarchy
# Another such section
You can also add non-section labels prefixed with l: (lowercase L + colon), with or without a link
l: Some label|http://some.link
anywhere in the hierarchy.
The hierarchy can be enabled/disabled by a setting in the configuration.
Two more settings allow you to apply changes to the section names and order from the block configuration.
Feedback would be appreciated.