Hello!
I've recently taken this old "AC Tips and Tricks" article that I came across and used it to create a public GitHub repository that contains code and documentation for creating custom workflow activities in Adobe Campaign Classic.
I thought I'd share it here, just in case it's of interest to anyone who is looking to extend their platform implementation in this way:
https://github.com/mroshaw/acc-custom-activity
I'd be very interested to hear your thoughts and feedback!