Not sure whether that answers your question; but the contents of sonar-project.properties are available to plugins if you pass a Settings instance to the constructor of whatever needs it.
This is due to the way SonarQube uses IoC; basically, ANYTHING implementing BatchComponent will be available as constructor parameters to classes on the "client side" of things, that is, basically, code analysis. And anything implementing ServerComponent is available if you write a server plugin.
Given that the contents of sonar-project.properties are automatically injected into a Settings instance, you want to add an instance of Settings in your plugin where it is necessary.
[side note: this is not obvious from the documentation alone; unfortunately]