I have implemented lots of WHMCS server provisioning module like you need. I have integrated lots of third party API with whmcs module.
I have been working in WHMCS and PHP since long.
Please check my proLisää
I have gone through your requirement. I can create this module to reboot dedicated server from client area by clients.
I have more than 11 years experience in PHP including 5 years experience in WHMCS. ILisää