This is basically because I can't call "pd open /path/to pathch.pd" with a relative path to the current canvas ...

i have not ever sent that message

wouldn't it be
pd open path/to pathch.pd

without the / in front of path

for instance

if your canvas is in a patch in home/patches/

and you wanted to load a patch from home/other/stuff/

;pd open ../other/stuff/patch.pd