Hmm I understand that limitation. But an attacker or a bad application can hide the important information which we need to identify the source of the problem.

What about limiting the fastcgi output to 1024 bytes and appending this info with max 1024 bytes.
client: 127.0.0.1, server: example.com, upstream: "fastcgi://unix:/var/run/php-fpm-example.com.sock:", host: "127.0.0.1" , request: "GET / HTTP/1.1"

[fastcgi - output max 1024][request info: client, server, upstream, host, request - max 1024]

This would ensure that client, server and upstream are always provided. Host and Request can be filled with "user generated" content, so you should put it to the end. This would ensure that an attacker cannot hide the important fields.