Okay, turns out that TFTP is a no go in my situation.
I hooked up Wireshark and saw that my client is getting through to the server just fine on port 69. I previously have also made sure that I have port forwarding on port 69 setup in my iptable
rules properly. Now I just read this on Wikipedia:
Data transfer is initiated on port 69, but the data transfer ports are chosen independently by the sender and receiver during initialization of the connection. The ports are chosen at random according to the parameters of the networking stack, typically from the range of ephemeral ports
i.e. TFTP won't work for me because I need NAT and it has to be secure. I'll need to go with a protocol that's connection orriented, ftp e.g.
I found that the standar VxWorks
library also contains ftpLib.h
(http://www.vxdev.com/docs/vx55man/vxworks/ref/ftpLib.html#ftpLs) that will hopefully resolve my NAT issues as FTP works with connection based TCP.