Have you found that other extension authors have this in mind? Most extensions don't seem to be built with hooks in mind so you're kinda stuck with either the existing extension or rolling your own. There also aren't very good primitives for most use cases because of this. I assume VSCode will get there though.
VSCode extensions are driven through "commands" that are exposed through the package.json of the plugin (which can be parsed by other plugins if you wanted) and issuable through another plugin, or using a task (this is kind of annoying though and not a first class use of tasks).
