I do not know the proftpd configuration, so I am shooting from the hip.
After you read over the proftpd configuration documentation, you may want to take a look at template fragments 10DefaultDenyWrite and 60AnonymousIBay in the /etc/e-smith/templates/etc/proftpd.conf directory. If it looks like something you can use, just copy it to the /etc/e-smith/templates-custom/etc/proftpd.conf directory before modifying it.
I also looked at 00Documentation which stated that ibays can only be written by admin or local users. Now I imagine the user must be from within the firewall to be considered local. But, anything can be configured the way you want it using the template system.
Good luck, and if you should find a soultion can you please write up a how-to document for others to benefit?

Tom