Gordon,
You mentioned D1.6 in the top post instead of D1.1 as mentioned for this one.
D1.6 also has provisions for prequalified WPSs, similar to D1.1. I'm just scratching the surface right now because I haven't got a lot of time at the moment. However the 1st part I turned to was section 3.15. That also points to figure 3.4 which has prequalified PJP square-groove butt joints.
You need to check other code provisions to make sure you comply with all applicable aspects, but it looks to me like you can use Fig.3.4 or 3.5 joints without qualification. If you need to prove penetration for some reason, I would weld a mock-up piece that is actual size and cross section the joints.
Chet Guilford