[mugwort]

Or I could try the roeselare yeast...

That's a delicious idea, all smileys aside.  Defridge & degas keg and pitch a fat starter.  Don't taste for 6 months.  Some amazing sours have come from last-ditch pitches.

[3bbrewing]

I like some of the suggestions of brewing a complimentary style on the dry side and blending the two.  I think that could work really well. 

Otherwise, I think souring it could be a great way to go.  If you already ferment in your kegs, leave it in the keg and degas it/let it go flat, then reinoculate it and put on whatever CO2 relief you use.  I'd try Wyeast 3763 Roeselare Ale Blend or WLP 655, 665, or 675.  On the downside, these all take some time to really mature.  You'll just have to brew something else to tide you over.