How to complete paths relative to the project root

[rcorre]

The path completion docs say:

Enable filepath completion. Show files and directories if an existing path at the cursor was recognized, either absolute or relative to the current opened document or current working directory (if the buffer is not yet saved). Defaults to true.

Is there a way to complete paths relative to the project root (or CWD) in a buffer that is saved? I find that most of the time, I want to complete paths relative to my project root rather than relative to the file I'm editing.