Marc,
There is a reason.
There is no market for building cues like that as long cues in general don't hit that well. They give a springy feedback with a standard diameter butt cap diameter, joint and tip sizes. They are too resilient. I guess if you made it thicker to scale with the same taper contours it would be ok and not so springy. They make tips up to 15 mm.
I am sure that Predetor or OB use standard 29 inch units to do their testing concerning deflection data and 30 inch is the production blank size. Maybe Royce at OB could help you on this one.
From what I understand, Harvy Martin always built his cue with his shafts longer than the butt . This gave his cue a little more joint to joint foot print at the face. If I were going to build a super long cue like that I would increase the thickness of the butt diameter and length and then make a longer shaft with a custom contour ending at 14 mm tip as a start point. You would have to do testing to see how it hits.
I am not trying to get the job of building a cue like that by any means, just commenting. I do not like to deviate from my standards but I am sure there are some more daring that will make your cue.
Have you ever played with a cue like that or are you just curious and want to try one that size?
JMO,
Rick