Now that siri shortcuts is out, I can enter spotify URIs and record a message for Siri to run it.
All that is missing is Spotify's support for URI that opens my saved "Songs" list and auto plays
Instructions for using Siri Shortcuts:
0) download Apple's free "Shortcuts" app from the appstore
1) create and edit a new shortcut
2) add a "URL" with a spotify URI as the content (ex: "spotify:user:XXXXX:playlists")
3) add an "Open URLs" object (causes url to open in safari, first time you run you have to confirm a dialog, but subsequent calls dont need to do this)
4) set a Siri Phrase to map a siri voice command to run this shortcut
note: if you run this from the lock screen, this will require you to unlock the device to complete
Updated on 2018-09-19
A similar idea has also been suggested here:
Please vote for it, and add a comment there, so it may be considered. Any update on the matter should be posted there.
For more info on how the Ideas Board works, click here. :)