Can you just use their Linksys box (with a $20 switch if needed), and enable the FTP fowarding you need in it?

It's either that or set a static IP on your Netgear WAN interface in the trusted linksys subnet - which should be different from your current trusted netgear subnet. Then set FTP forwarding from your Linksys to the Netgear, and it should continue from the Netgear to the FTP machine, doubly NAT'ed.

-jk