I had a "Rule of Cool" situation last year in a Pathfinder game I was playing in. The set-up:
The party was in a dilapitated mansion that held hundreds of mirrors hanging within it, each of which was more like a "window" that looked through into a room and one could see an individual moving and walking around inside that image (a la the paintings at Hogwarts". Now there's no rules in the game specifically designed for this effect, the DM just had to make some up. One of the rules he invented for the narrative of this adventure location was that if you touched the "glass" of the mirror, the image of the person changed, they turned into some sort of weird undead creature, and "climbed out" through the mirror to attack us (a la The Ring). Again, there's no rules in the game that explain these effects, the DM just made them up to use for the encounters.
I was playing a Paladin and while we were in a particular part of the mansion in some other fight, one of the PCs accidentally fell backwards (probably via a Push effect) and collided with a mirror, which then triggered one of these "mansion image ghosts" to turn evil and crawl out of the mirror to attack us.
Now I as a Paladin had no abilities that would allow me to interact with the actual game rule of "incorporeal" and thus be able to interact with this ghost physically as per the "game rules". However, on my turn I asked the DM if I was to use one of my Lay On Hands uses (that would allow me to damage undead per the rules) as well as a use of Smite Evil (that allows me a greater effect versus undead, fiends and the like) and combine them together to let the "energy of the divine magic create a solid force with which to shove the incorporeal ghost back into the mirror?"
Again... the rules do not allow for this. There's nothing that says using Lay On Hand and Smite Evil combined gives you the ability to affect incorporeal creatures. But the DM thought that this sounded like a cool thing to do, especially as I was willing giving up two uses of my paladin abilities to try it. So he said "let's try it!" and had me make a Strength check against the ghost to see if I could it back 5 feet and back into the mirror. I succeeded in doing so, and once the ghost was back on the "other side" of the glass... it turned back into it's normal image form of a person inside their bedroom.
None of this stuff could be considered Rules As Written, nor Rules As Intended. It was really just this DM using his "DM Fiat" to decide that what I was trying to do (and more importantly what I was giving up in paladin use abilities) was "cool" enough to let me try. And it ended up working out and created a whole new narrative direction that this adventure allowed us to go.
That's what the "Rule of Cool" can get us... whether or not you wish to call it the "Rule of Cool" or "DM Fiat" or "DM adjudication" or whatever term you are or are not comfortable with. Merely the opportunity to do things that aren't written down in the books as a hand-and-fast rule.