Ok Snook,
I'm not sure how useful the posts so far have been. Some suggestions contradicted exactly to what I believe is true.
Tele lenses give great compression, little distortion. But for maximum long legs I take wide-angles and get really close and very low (not just hip height but almost touching floor with the ear level). The wider the better (more distortion). So your 65mm is probably not the worst choice (although I own the 50mm RZ lens not the 65mm).
Now why doesn't it work? Well the best (worst) part of the lens is outside of your very cropped (in regards to 6x7 format) P30 frame. The further away from the center the more distortion the lens has and enlongates legs and such. The center of the wide-angle is more neutral in a way (which seems often to make matters worse compared to the distorted areas) and because of your cropped sensor your frame.
Have you tried to shoot the P30 on a 645 body with a wide angle? That would cover more of the outmost areas of the lens.
How I'm doing it:
Unretouched pic I shot at a workshop I gave (model about 6'):Canon 1Ds MK3, 24-70mm 2.8L, used at 24mm
From as low as possible, with feet as close as possible to the lower edge
The shorter the skirt/dress the longer the legs :-)
The skinnier the model the longer legs/arms will appear.
I find it important that the chin/face is not covered by the shoulder that can easily happen. The model shouldn't lean back from the camera with the upper body as this will make the head/upper body even smaller.
Click to view attachmentQuickly retouched pic to give the model something for the bookFace replaced from a different frame
CTRL + T to TRANSFORM
Move the middle point to the lower edge or the face (depending what way you go)
Now press and hold the CTRL to DISTORT, pick the left top corner and drag it up (and possibly a bit to the left too), take the right top corner and drag it up and maybe a bit to the right
Sometimes this will distort parts of the image for the worse (wider hips, smaller heads). That's why you may feel the need to move the top corners more to the outside to counter-act.
Another way to minimize adverse effects is Edit/Transform/Warp. This lets you bend move parts a bit. But the controls are rough and not super flexible
So now the LIQUIFY filter comes in.
You can use it to further reduce adverse effects and make legs more shaped, hips tighter, head bigger/smaller/longer/whatever, feet smaller, etc.
This is the finished pic:
Click to view attachmentAnother sample with a 5'6" model:Click to view attachmentSometimes the effect is too strong so I will back off and used a bit a longer lens. But generally speaking, the closer and lower down (touching the floor with the ear) you are and the closer the feet are to the lower edge of the frame the more extreme. Not sure if your 65mm on the RZ will work for this though.
I don't know why but when uploading the pics to the forum here suddenly they have bad posterization and a bit of a color shift. But anyway... the point was enlongating the legs.