What is the reason ngx_http_fastcgi_module doesn't support subrequest_in_memory flag?

What is the reason ngx_http_fastcgi_module doesn’t support
subrequest_in_memory flag? What is the best way to do the same this for
fastcgi? Set body filter and then empty buffers in incoming chains?

Posted at Nginx Forum:
http://forum.nginx.org/read.php?2,223073,223073#msg-223073