Great, thanks for the explanation, this answered my question!
So from that I can conclude:
1. For my custom tab, if I just name the link of the CRM_NewActivity screen after one of the links in your default tab, ie "Log Call" or "New Task", then the hidden button progrmaming will kick in automatically and turn it into the screen I want.
2. Programming a OnLoadAppearance button (does it have to be a button? I'm guessing not) can be used as a way to change the default values of fields on any of my screens. Which is cool.