I suspect that your packets have to look enough like TCP to keep the
NAT box happy, but that doesn't mean that you have to do either
congestion control or retransmissions.   Very hare-brained scheme
follows:

You could add a sockopt to have TCP not look at congestion window, and
only keep the most recent segment around to retransmit.  You might
still respect the window, and use retransmission of the one segment to
be sure you get a window ack.

Force every transmitted segment to be a whole packet (MTU?).
Or add a pseudo-header with packet number and fragmentation, so you
can reassemble arbitrary packets.

On the receiving side, ack every packet with its own sequence number,
and process what you get.

This would act essentially like UDP, but look enough like TCP to the
NAT box (since it probably isn't scoring your congestion control
fairness).


-- 
        Greg Troxel <gdt@ir.bbn.com>