The install of Karabiner should have come with Karabiner-EventViewer as well.
If you try typing the pipe on a normal ANSI or ISO keyboard, it will come through as "backslash" with the flag:"left_shift" or "right_shift"
If you are okay with that key, then simply rebind a key to "backslash" and then use shift with that key.
If you prefer to only be able to type pipe and not backslash, then you will need to write a complex modification that replaces a specific key with
- that key, when used without shift
- "backslash", when used with shift.