OK, I'm half way done (pics in Fran's Stuff), and decided to take out the waste canvas threads in the finished half. Turned out to be a bit more of a bear that I thought, but they did come out, even from under the over-one stitching.
Two key learning's: 1) You will have to get the material very damp (but not dripping) in order to loosen the starch, and the starch sets back up when dry, so you have to keep it damp as you work.
2) Since I am working with Silk on silk, both need to be washable because the starch will transfer from the canvas to the silk as you work to take the canvas out. This means I will have to wash the piece afterward.
A note about silk dyes - traditionally people have not used mordants when working with silk dyes, which is why they will spot or bleed when washed. Mordants to fix the dyes have been available, but they tended to severely weaken the fabric and so were not extensively used. Modern washable silks have had the dyes semi-fixed with modern mordants which do not cause as much weakness. However, they will still fade considerably with multiple washings, which is why most silk is labeled as dry-clean only.
Fran