Depending on the specifics, you might be able to add socat in the middle.

Instead of: your_app —> server

you’d have: your_app -> localhost_socat -> server

socat has command line options for setting tcp_nodelay. You’d need to convince your closed source app to connect to localhost, though. But if it’s doing a dns lookup, you could probably convince it to connect to localhost with an /etc/hosts entry

Since your app would be talking to socat over a local socket, the app’s tcp_nodelay wouldn’t have any effect.