Easy! A 29er hartail, just like my Santa Cruz Highball of course
Here's a pic of actually commuting to work too!
With a lockout fork, of course, as you say. It's fast and light enough for XC. Did a great job leading a few laps of TwentyFour12 24hr solo (until I swapped bikes with a puncture and the rear mech exploded on the other).
I wouldn't bother with a second set of wheels and tyres, just pop a fast rolling tyre on the back, something like a Crossmark or a Saguaro with a reasonably solid centre rear tread. My experience is the faff of swapping cassettes, wheels, re-aligning brakes isn't worth it and the knobblies stay on. The cost of a full wheelset, spare discs, tyres, cassette pays for a lot of new rear tyres