Steam blocking a/k/a "killing" acrylic really does work.
I took Elizabeth's very good advice and practiced on a swatch first, then moved to the real thing. It was still slightly unnerving, hovering a steaming iron over the piece, If the iron had actually touched the fabric it would have been melty goo and a ruined project. I worked a section at a time, standing at the ironing board, smoothing out a section and lightly steaming it, then pinning a bit more stretched and giving it some good hearty shots.
It grew considerably in size. I don't know how much (if any) of that was actual stretching of the fiber and how much was just the lace opening up.
It was an interesting experiment.
Now, as for what I might do for the third shawl of the year, I haven't a clue.