It looks like it's controlled via the tooltypes. From the docs:
ExternalARexxCMD=comm. Use this user defined ARexx command. The command should
contain %s as placeholder for the full path name of the
pictures.
Example: ExternalARexxCMD="address command 'cybershow %s bm sl'"
can (with REXX as port) be used to start a shell command,
in this case CyberShow in BestMode&SlideShow mode.
Please note: Currently PhotoAlbum will be blocked until
the command returns.
So, this may work:
ExternalARexxCMD="address command 'multiview %s'"
Edit: You probably need to use the run command because Multiview does not return until it is closed (see note above from docs). So, try "address command 'run >NIL: multiview %s'"
