The Jahnd (sp) racks of LowRider style sit higher on the fork than the original Blackburns. I've never had the rear Ortleibs, just the smaller fronts but I am guessing you could run the rears on the Jahnd's as long as you accept that in taking hard corners, they will drag. (Ortleibs take that kind of abuse on an occasional basis very well, abssorbing road rash far better than I do!)

I am a huge fan of LowRiders and putting weight there. Actually makes bikes easier to ride and control in snow and other iffy surfaces as well as not affecting the bike or steering at all when you come out of the saddle to climb. The piece that is different is that the front tire and rim get far more abuse and need to be bigger with more pressure (the tire) and strong (the rim and build).

Ben