Is there a way to create a magic-mouse double-tap gesture on a standard mouse? Something like clicking a middle button on the mouse to trigger a double-tap?
There is the predefined action "Smart Zoom", which allows you to trigger the default zooming that happens when you do a two finger double tap.
Is that what you need?
What I'm actually looking for is the app-specific expose you get when you double-tap an icon on the dock. I'd like to see the apps open for a specific application with one tap.
you can use the predefined action „application expose“ for that 
(not sure about the exact name, I‘m currently replying from my phone)