/proc/$PID/exe
parece ser o que você está procurando: ( proc(5)
/proc/[pid]/exe
Under Linux 2.2 and later, this file is a symbolic link containing the actual pathname of the executed command. This symbolic link can be dereferenced normally; attempting to open it will open the executable.
Então, simplesmente:
$ /bin/cat & readlink /proc/$!/exe
/bin/cat
Na verdade, segue as renomeações no arquivo executável:
/tmp$ cp /bin/cat . ; ./cat & mv cat dog ; readlink /proc/$!/exe
/tmp/dog