Heidi Klum has come up with 2 maternity lines found at Destination Maternity. They each have some tall maternity clothes. On the left is a Lavish By Heidi Klum mock layer maternity tank top, a nice basic piece for tall women.  On the right is a Loved By Heidi Klum empire maternity maxi dress always a nice way to get a longer length maternity dress.

Long Tall Sally Tall Maternity Jeans 36" Inseam
Stretch denim tall maternity jeans, bootcut.
Wide elasticized waistband for extra support.
Mock fly front and back pocket stitch detail.
Sizes 6-16 $79

Ontario Tall Maternity Pants

36" inseam. Wide leg styling. Jersey insert front and hook and bar fastening. Sizes 4-18 $72

RG Maternity carries tall maternity pants and jeans with 36-37" inseam lengths in sizes ST-2XLT. They also have long torso maternity shirts.

Moms Maternity carries tall and tall plus size maternity pants with 37" inseams in sizes 4T-26T.