Custom ContentProvider, HierarchicalList and "for this page" folder

I'm having difficulties using my own ContentProvider and setting up the "For this page" folder in the assets pane in the editor view.

TL;DR
Using widget epi-cms.widget.HierarchicalList I can't see "For this page" folder in assets pane
Using widget epi-cms.component.SharedBlock I CAN see "For this page" folder in assets pane but my custom ContentProvider is not beeing used

I have a database full of workshop-entities that need to be editable via the EPi UI (create, edit, delete). Every workshop is assigned to a page in episerver. So I created a custom contentprovider to serve the workshops from the DB to Episerver.

I map my Workshop entities to WorkshopContent (that inherits from ContentBase, IContent) and everything is fine.

So far so good.

The problem I'm having is that I'm never able to access or see the "For this page" folder in the assets pane

In my ContentDescriptor I have tried setting all different kind of Roots but the only one that works is the Entry-point for the ContentProvider.
I have understood leaving it (the overridden Roots property) blank (or returning empty array or array with empty ContentReference..) would render the "For this page" folder but alas.

This is all using my custom ComponentDefinition specifying widget type epi-cms.widget.HierarchicalList

When using the epi-cms.component.SharedBlocks widget-type I DO see the "For this page" folder but then my ContentProvider is not beeing used (its creating my WorkshopContent using the default content provider).

I have tried following the following guides

1. http://world.episerver.com/forum/developer-forum/-EPiServer-75-CMS/Thread-Container/2014/9/Custom-IContent-type-with-asset-pane-wont-show-For-this-X/
1. And its linked blogpost
2. https://github.com/episerver/YouTubeContentProvider/
3. http://world.episerver.com/blogs/Per-Magne-Skuseth/Dates/2014/11/content-providers-101--part-i-introduction-initialization-ui--identity-mapping/
4. http://jondjones.com/how-to-display-non-episerver-content-within-the-episerver-editor/

Would I need to create a custom DOJO-widget for this to succeed?

Whats up with the relationship between the properties ContentProviderElement.EntryPointString and the ContentRepositoryDescriptor.Roots?

Does anybody know about anyother documentation or guide out there?

Thanks in advance!