0x19+size seems wrong, maybe it should still be hello, then do %#x-rip?

memcpy PIE is done here with the magic _PKST_ label: https://github.com/lunixbochs/patchkit/blob/master/core/arch.py#L73

That's exactly what I was looking for. Updated as you suggested, looks much better.
Tested with PIE and standard ls binary

It is a shame that the hello variable isn't used though, it's nice to see that inject returns that address and how to use it. Maybe it's good to have this as a separate example, hello64_pie.py?