Would it be possible to add something in the interim, before a full plugin API is worked out. I am thinking of a plugin folder and configuration variable to hold that location of the folder. Template files for one, could reference any extra code they needed by using something along the lines of ...
Well template files should keep their needed code inside their own template directory. I think some method to plug into the parser/handler/renderer structure will be implemented first (to replace the old customs.conf) - but I think I will do a release before this is added.
Syntax plugins are partly done for the current devel version.
I think this task can be closed. The syntax and admin plugin API works well and there are already plenty of plugins.