SharePoint designer has a great way to create custom actions. There are many help posts out there on how to do that.
You have three options to select "Navigate to form", "Initiate workflow" and "Navigate to URL"
The trouble with these actions and options (well the ones on the View Ribbon) is that they do not open in the dialog box
There a heaps more posts on creating custom actions that open in dialogs
The problem with those though, is that the action must be defined before hand.
I really like how sharepoint designer allows you to configure the url on the fly.
There must be a way to combine the two ways.
I was hoping that the "Navigate to URL" option would be a href.
Sure enough i got an alert box.
From there it was just a matter of figuring out the showModalDialog command, and putting it on one line.
This action is great to use with the StartWorkflow component http://spc3.codeplex.com/
For View Ribbon actions
And for Display Form Ribbon actions