Coasting? Sure, but most people with a "plug in hybrid" don't like it when their gas engine comes alive with the slightest press of the gas pedal, which is probably close to what BMW will have to do with a mere 25-50HP on tap.
With that said, spy photos show that BMW does have a plugin hybrid 5er in the works -- it seems like they're way ahead of us in exploring down this path
The AH3/5's drivetrain is designed in a way that lends towards a really good PHEV implementation that can be achieved "just" by making the battery and motor bigger. And I'm sure when BMW thought of this car 5-ish years ago, they had that in mind as well.