Hi there, so here's the situation.

Computer 1: a raspberry pi with no screen, keyboard or mouse that automatically runs a headless instance of pure data when it boots up: pd -nogui mypatch.pd

Computer 2: my laptop. I want to start the GUI separetly and connect it to the existing pd instance on the raspberry pi so that I can program the patch, alter it, save it, and watch it in action. I believe the command should be something like: pd-gui HOST:PORT

But I can't seem to make it work. and there is no documentation to speak of anywhere. Can anyone help?

Why not SSH into the Pi like this:
ssh -X pi@ip_address
then run:
sudo killall pd
to kill the active Pd process, and then launch Pd with its GUI with your laptop as the host?

@alexandros That will work, but it would interrupt the sound. :/

If you want to do it during the audio process, then maybe OSC would be a good solution. You could open a new Pd instance in your laptop with the same GUI of the patch running in the Pi, and send all control data via OSC... just a though.

@jeremiahrose Hello Jeremiah,
I found (for me) the best way to run my pi headless is simply to VNC "into it".
I have this file on the pi....... set to run at boot....... X11VNC.desktop
using desktop "0" so that there is no second instance of the desktop running on the Pi..
There is more info for setting this up on the web than there was when I did it years ago........
http://blog.tonywall.co/2013/07/setting-up-raspberry-pi-for-headless-mode-with-x11vnc/
Then, as I am using windows I use TightVNC on my laptop........ and I simply use my laptop screen, monitor and mouse with the Pi as a window. When I drop the connection the Pi is still running everything, and I can pick it up again when I wish. Pd on the Pi doesn't care because I am only addressing it through the Pi desktop.
I still use putty for sudo stuff, and a samba share for non root file transfers.
David..