So, I've gotten really close to replicating a click-to-cast addon in Rift by setting up an action bar just for the spells I want to bind to mouse buttons. For example, I make a macro like this:

cast @mouseover Restorative Flame

Then I put it on my special action bar just for this purpose, then I open the Key Bindings interface and bind that slot of that action bar to the combination of mouse buttons and modifier keys that I want.

It's ALMOST perfect except for one little problem. The key down event (or whatever) for the modifier keys (Shift, Ctrl, Alt) etc. gets consumed on the first cast. So if I have a spell bound to Shift + Mouse Button 4, it will work once, and then whatever is bound to Mouse Button 4 will be used until I lift the Shift key and press it again.

Oh if only that wasn't the case... =(