Just so you know, I haven't forgotten about this WHMCS issue. The problem is that I don't think WHMCS allows us to present a password field to clients during the WHMCS signup process, so it may be tricky to work around this problem. We may have to resort to a plaintext input field, which is rather less than ideal.
I'm having some troubles accessing my developer account with WHMCS, though, so I can't check their SDK at the moment... will update when I've managed to get in.