I love that fabric!
Could you show close up pics of the lining, hand stitching, seams or hook and eye closures? Any or all of these would help greatly in dating it.
My first thought is a stage costume as well. But it really has a lot more detail then a costume would require. Maybe it is for reenactment, I believe they would be looking for more detail in the costumes they use.
Is any of the silk shattering anywhere? Any staining to the bodice lining under the arms?
Any way you look at it it is such a pretty dress.
Kat