Yes, very interesting. I've never seen one with anything like that.
I suspect it was added by someone themselves (rather than a prototype), since the artwork (especially at the outlanes) doesn't seem to support it.
If this was something that was being considered early on, then I would expect the artwork (and outlane inserts) would have been positioned better to support it.
On a side note, during last night's Monday Night Football on ESPN, they showed a brief segment on the Seattle Pinball Museum, with a WhiteWater prominently in view (though they mostly highlighted the Monday Night Football machine).