A comment here from Evan made me realise that there doesn't really exist a good reference for property paths in SSIS. Property paths are used in configurations and the /SET option of dtexec.exe to set properties in the package dynamically at runtime.
For some inexplicable reason dtexecui.exe does not contain a user interface for building these things yourself so you have to manually encode them yourself. This is a major shortcoming given that they could easily lift the package browser from the Configurations Manager:
If you want to change this, vote here and ADD A COMMENT: https://connect.microsoft.com/SQLServer/feedback/ViewFeedback.aspx?FeedbackID=124577
Anyway in the meantime, until Microsoft give us a package browser in dtexecui.exe, I thought it would be useful to list a few myself. This is as much for my own reference as much as anyone else's.
\Package\Data Flow Task.Properties[[DataReader Source].[SqlCommand]]
Hopefully that should be enough of a reference to be going on with. If I've missed anything out, let me know!