Either create a separate ftp service that uses whole other set of text files. (Copy the default ones to new dir and modify at will)
Code:
[FTP_Service_Temp]
..
Messages = ..\text\ftptemp
Or restrict access to sensitive information in /text/ftp/welcome using the include cookie. Place the info in a file and set permissions for who will be able to see it in welcome message at login.
Code:
%[include(..\text\ftp\welcome.info)(MG13)]
That for example will show the contents of welcome.info to all users with flags M, G, 1 or 3 (regular users). For temorary users simply remove the default 3 flag.