Ah, that is STDIN ending, which means EOF.
I think that what you would like is the existing capability except instead of ending when STDIN ends, it is open ended and continues as an interactive session after stdin is exhausted.
Well, this is getting more complicated ...
If I start FTP with stdin redirected from a file and switch to using the keyboard interactivel using BIOS calls, it works fine. So this lets me start with a script, run a few commands, and then move to interactive use. Which is what I think you are asking for.
But if you shell to DOS, it gets wedged. DOS things that EOF on stdin has been hit (and it has), and you can't type anymore. The only way to escape is to reboot. I tried clearing the EOF flag using the C runtime but that didn't work.
So I'm thinking now that the only way to do this is to provide a command line parameter that says 'run this script' instead of redirecting from stdin. I think that is the only way to do it without having to hit DOS on the head with a hammer and tell it that STDIN is not really gone.