At the moment the Sharepoint doesn't support Lists which contains Calendar Items.
So in my opinion the Sharepoint Feature in PowerApps is nearly useless if it doesn't show all lists.
If you could implement support for this lists it would be great.
I too would like to see this happen. It would let me build somthing similar to below
I got a SharePoint Online (O365) List that is a Calendar to work in a Flow. I have an approval list that then enters data from the request list into the Calendar list. When setting up, I used SharePoint - Create Item Flow activity and had to manually type in the List Name once paste in the URL of the SharePoint Online site (10/25/16 confirmed working).
However, not all the fields are available - for example, the All Day Event field is missing, but Event Type (used for recurring events, normally a hidden Calendar column) is visible.