I'm the lead dev of ApisCP. If this consideration gains further traction please feel free to reach out to me at
[email protected] if you have any questions in developing the module. Blesta was done over a weekend and WHMCS didn't take very long to flesh out. I'm more than happy to address or even too extend ApisCP's API to facilitate whatever needs to be done.
Edit: this username was brought to you by autofill 😬