[b][color=#FF0000]UPDATE[/color][/b]

Now, it is possible to do this way:

[code]
fastcgi_param PHP_VALUE "upload_max_filesize=5M";
[/code]

More information here:
http://bugs.php.net/bug.php?id=51595

Using this technique, I have successfully set different "error_log" locations for multiple virtual hosts.

Thanks, PHP and NginX guys!