While my wife is not an organist, in the symphony she has chosen black pants which are made of material that is quite flowing (silk-like), and appears dress-like unless viewed straight on. This allows for controlling the direction of the "hang" with the legs, as well as preserving the appearance of a full-length orchestra gown. She tells me that it is two layers. The first layer is the actual pants, and the second layer is the semi-transparent material which gives the illusion of being a dress. She says she purchased hers too long, but varying lengths should be available.
Hope this helps.
Michael
Allen Organs (505-B & ADC-6000), Frazee Pipe Organ (2/13 w/chimes),
Pump Organs (Estey, Sears & Roebuck, Mason & Hamlin, Chicago Cottage, Williams & Sons, Angelius, Cornish)
Pianos (Ivers sq. grand ca.1865, Ivers & Pond Upright-1929, Technics SX-PR600)