I did some research and measuring on this possiblity a while back, and as near as I could tell, the risers are on 3.5" centers just like any other Harley Davidson bike. The bolts that come up from the top triple tree are metric theaded, but most aftermarket bars are taped out to 1/2" course threads. The empty holes in the triple tree don't care what kind of bolt goes through them.
They only probldem, and it can be a significant one, is fitting the aftermarket bars into the "clam shell" speedo cluster on our bikes.....but, I believe there are some out there that will work. I believe you need 5" to 6" risers with no more than a 3" pull back. I have not actually done this swap yet, but may do it some day soon. I even think you can use the individual risers with a set of wide or drag-type bars... if..the risers fit in the cluster with enough room for the clamps that hold the bars on. Otherwise, there are lots of bar/riser (all-in-one) sets available in the Custom Chrome, or other catalogs.
Good Luck, redrazor