Reproduced by either:
right-click $dir > 7-Zip > Add to Archive ... > Create SFX archive
p7zipForFilemanager ad -t7z $dir
Output (GUI dialog):
No such file or directory
7-Zip cannot find specified SFX module
However, the file exists:
$ file /usr/lib/p7zip/7z.sfx
/usr/lib/p7zip/7z.sfx: symbolic link to `7zCon.sfx'
$ file /usr/lib/p7zip/7zCon.sfx
/usr/lib/p7zip/7zCon.sfx: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.27, stripped
Removing the symlink and copying 7zCon.sfx to 7z.sfx has no effect and results in the same error.
Using the commandline directly with 7z binary and creating an sfx with '7z a -sfx $archive $dir' successfully creates a binary $archive, so the sfx module is indeed usable by p7zip in its core functionality.
Problem lies in the GUI implementation, in particular, 7zG itself or how p7zipForFilemanager handles it.