I don't know what your reason is for disallowing access to the z/OS FTP server but I suspect it is security-related. The z/OS FTP server is not well protected, especially when the z/OS system is Internet-facing. I have a z/OS based software product for monitoring and securing the z/OS FTP server that I would be willing to donate a license to if this would make allowing FTP server access a possibility.
The product is called FTP/WatchDog-Z. Information about the product can be found at www.softwareassist.net.
With WatchDog, you would be able to monitor z/OS FTP server usage in real-time, block unwanted FTP server activity with RACF rules and automatically detect and shut down z/OS FTP server attacks.
If you are interested, please let me know. Thanks.