There is no way to promote a grid master candidate to a grid master via either the Web API (WAPI) or the older Perl API (PAPI). The problem is that traditionally only the Grid Master has been able to receive and process API calls. So if the Grid Master were down or unreachable then by definition there was no API access, and hence no possible way to use the API to promote a Grid Master Candidate to be the GM. Promotion has historically been done via the command line interface because the console CLI interface was the only interface guaranteed to be available on the GMC. (The CLI can be made available via SSH as well, of course, but this is an option that needs to be enabled.)
Recently NIOS has been modified to allow read-only API access to a GMC, as well as read/write API access via the Cloud Platform appliances. So it's at least possible in theory that one day NIOS might be modified to allow GMC-to-GM promotion via an API call. If you are interested in this capability please work with your account manager and SE to submit a formal request for enhancement. (I just checked, and could not find any existing RFE that discussed this proposed feature.)
