How about a sink/pressure mount?
Rigid aluminum honeycomb backboard (SmallCorp), Mylar tracing made of dress's outline and used as a guide to cut a 3-D sink mount from layers of foamboard or Ethafoam etc. faced with good quality matboard and covered with needlepunched polyester padding and then a beautiful washed silk or some such dress fabric.
From bottom to top: honeycomb panel, foam sink mount faced with matboard, polyester padding, show fabric.
The wedding dress (padded where need be) is nestled into it's custom made sinkmount. The idea of the 3-D sink mount is to have the bottom raised and lowered where necessary, so that the front surface of the dress lies more or less on one plane. Slight variations are taken care of by the puffiness of the underlying padding.
Tacking stitches can be made in strategic places using a curved needle to hold loose bits in place.
Acrylic glazing is placed on top touching, but not squashing, the dress. The package is slipped into a deep sided frame (Small Corp makes powdercoated Al frames with welded corners) and a basswood strainer is placed at the back. Sides of the frame are screwed into the strainer, holding all together.
This is a broad stroke description - details on request.
This probably won't work if the dress has a very structured bodice though.
Or, you could just order a dress form and display case!
Rebecca