Does Tomcat 4.x.x do Explicit Congestion Notification (ECN)?
To do this you would have to set the IP header's CE (Congestion Experience) bit and I can't see any way to do this. To me ECN seems to be a pure low-level network way to deal with bandwidth problems.
If your Tomcat server wants to send data to the client, your server's IP stack determines the speed for tomcat. Tomcat just waits for the socket to send more data.
Unfortunately this is no option for a router. It's got some RAM to buffer packets for a while and if it's insufficient it has to drop packets and inform its neighbours by ECN.