I have a very simplistic patch for netcat here:
http://home.sch.bme.hu/~piglet/netfilter/tproxy/netcat-ip_transparent- support.patch
I think I came across that patch before. The problem is I can not find that version (1.10) of netcat anywhere. Found some places that mention being pointers to it, but dead links... It appears there a new GNU version that has lower version number, and also a BSD version, but it is way different too (or at least different enough that the patch doesn't apply).
Reusing the original port is usually a bad idea. A notable example of things breaking is Netfilter connection tracking, which gets confused if you reuse the exact same endpoints for a different connection.
Technically they are not the exact same if you include the interface. If it doesn't consider the interface then they would appear the same.