Unable to use variables in error_log

[Phill]

It seems as though its not currently possible to use variables in 
error_log config. At the moment I use a config similar to:

set $root /var/www/myapp;
root $root;
location ~ \.php($|/) {
set $script $uri;
fastcgi_param SCRIPT_FILENAME $root$script;
}

I'd like to add:

error_log $root/logs/error.log;

but that tries to write to "\$root/logs/error.log" rather than the 
expanded "/var/www/myapp/logs/error.log".

Will this be available in future releases?