There's no way to do that in FTP. I'd have to do it with something like SSH, which means (1) knowing the OS the ftp server is running, (2) logging in as root (or at least some kind of admin user), (3) navigating to the appropriate directory, and (4) running the appropriate chmod/attrib/acl/whatever command.
That's not likely to happen. But feel free to implement it as a plugin!
Unlikely. /S sends commands to FTP servers, not to the underlying Linux / Windows / whatever server. Unless you know what FTP software the server is running, and that it will pass those commands on to the OS (which would be a really really bad idea if it was a public server!).
It might be ... via the SITE command. I googled a couple examples of "SITE CHMOD ...". As an anonymous user, I don't have access to prospero's SITE command.
v:\> iftp /v anonymous:email@example.com
Connecting to FTP server.
220 (vsFTPd 3.0.2)
331 Please specify the password.
230 Login successful.
v:\> iftp /s help
214-The following commands are recognized.
ABOR ACCT ALLO APPE CDUP CWD DELE EPRT EPSV FEAT HELP LIST MDTM MKD
MODE NLST NOOP OPTS PASS PASV PORT PWD QUIT REIN REST RETR RMD RNFR
RNTO SITE SIZE SMNT STAT STOR STOU STRU SYST TYPE USER XCUP XCWD XMKD
214 Help OK.
v:\> iftp /s site
550 Permission denied.
TCC: FTP protocol error: 550 Permission denied. "site"