What you would need to do is on the server you want to read off the load is make up a script that reads the load from the server and displays it. Then you would use cURL to read that page that displays the load.
For that script to read load the easiest way would be to use SNMP to read the servers MIB's. With this you can get all sorts of info such as server load, uptime, disk space usage and network utilization.
If you used SNMP you could cut out the cURL bit as SNMP can be read from a different server providing the traffic isnt blocked.
Take a look at SNMP over at PHP
PHP: SNMP - Manual
Also I have some sample code to read disk space usage with SNMP on my blog
Brady Getting Windows HDD usage with PHP and SNMP
This is only one way of reading system data and I believe there are others.
Hope this helps