DC seems to support account configuration via QR code. see: mailadm: managing token-based temporary e-mail accounts — mailadm 1.0.0 documentation
this docs talks about temporary accounts, but it has not to be.
you can generate a QR code containing an url which yields back an mailbox config. it’s more or less the same as your workflow. you only need to give QR codes to the users and have a server side script create mailboxes.
see also here [Manual] QR code (tips and tricks), and bugs?