I use curl and the Roku external controller API to lauch Spotify.  It looks like I should be able to specify additional paramaters when launching the channel to include a playlist.

The following works to launch the app and start playing a playlist.

% curl  -X POST -A '' -I http://192.168.0.192:8060/launch/19977
 
# Then I wait about 5 seconds before pressing the down and play buttons.

% curl  -X POST -A '' -I http://192.168.0.192:8060/keypress/Down
% curl  -X POST -A '' -I http://192.168.0.192:8060/keypress/Play

According to the Roku docs (http://sdkdocs.roku.com/plugins/viewsource/viewpagesrc.action?pageId=3113168) I should be able to  specify additional options.  I think these would be specific for the spotify app.

I am hoping that something like the command below would launch the app and start playlist 1:

% curl  -X POST -A '' -I http://192.168.0.192:8060/launch/19977?content_ID=playlist1