I had some conversations with support at ZAdomains support about this and mentioned that the API only has Domain_Delete,Domain_Select, Domain_SelectAll and Domain_SelectAll_ByContact .
I am not a developer, but I understand it would need actions such as:
Register Domain
Transfer Domain
Renew Domain
Release Domain
Delete Domain
Modify Contact Details
Get EPP Code
Get/Save DNS Records
Get/Modify Nameservers
Register Nameserver
Modify Nameserver
Delete Nameserver
Get/Save Email Forwarding
Domain Cron Synchronisation
Your WHMCS API uses ioncube and I couldn't glean much from it.
As Mike said, you would certainly benefit from having a presence with Blesta. As far as I am aware there are currently no other co.za registrars that have modules with Blesta. You could be the first.