I am considering forking Parcellite and making it use AppIndicators now that Ubuntu has dropped support for systray. How different are these, and are there any "getting started" guides on AppIndicators yet?
It's not really an answer to the question, but a remark about the point of Application Indicators.
You actually do not migrate from System Tray to Application Indicators. These are different things and you shouldn't use indicators for all the various things you used tray for. The purpose of indicators was to stop the malicious exploitation of tray and you shouldn't try to migrate this exploitation to indicators.
Take your time and read carefully and with meaning the paragraph dealing with this issue:
API documentation is available here -
Some information are also avaiable at Ubuntu Wiki -- https://wiki.ubuntu.com/DesktopExperienceTeam/ApplicationIndicators#Technical%20Resources