ServerPing supports creating a product/service and restricting the checks that a customer can add to only those listed in WHMCS. With this option enabled, when a users adds/edits a check, ServerPing will resolve the hostname/url then check to see if it is listed in any of the clients services under the Dedicated IP or the Assigned IPs fields. If the ip is not listed in either of these two fields, then an error will be returned.
To use this feature, create a new package by following the normal instructions:
http://www.serverping.net/clients/knowledgebase/2/Adding-ServerPing-Package.html
However, when setting up the Product/Service on the Module Settings Tab, check the box, Restrict Checks to Assigned IP addresses to enable this feature.