>Then include the port number in the hash function.


No. We can't do that, it violates RFC-791 (and thus  RFC-1122,
per the  references to RFC-791 on fragmentation and reassembly).

If you meant "IP protocol" rather than port, that doesnt help my
scenario one iota: assume all traffic is UDP as the worst-case.


If you meant "port": "Port" is a transport-level notion. Transport
protocol headers only appear in the first fragment of a fragmented
datagram.  A moment's thought would show that if a receiver sees a
fragment other than the first, it has no idea which "port" to
associate that ip_id with, and so no way to pick the correect fragment
chain.