This is what I ended up with.
As you can see, the idea expanded a bit. Still some water effects to go on, but other than that, done. When I was digging through my bits box looking for interesting little bits for it, I thought hey, why don't I make it into more of a floating slab of rock magically torn from the bog. Then I could use it to give the meargh some height when she's pretending to be a daemon prince, or I could even magnetise the top so I could put other fimir characters there if I needed a disk rider. So that's the way it went.
One thing I am thinking of is to just add a permanent rider to the plinth. A dirach, possibly. Then I could add some paraphernalia to the rock, a familiar, spell book or some other bits. The familiar would be especially appropriate for a dirach. Here's the model I'd be thinking of using, from Oakbound games. He'd need a little conversion, some horns and perhaps a weapon, but other than that he's pretty much bang on. His pose would work well as a rider.
I'd then work up the original base extender for the meargh, and lo, I'd have the meargh and a new mounted character.
Or, maybe I'm just making work for myself.