I dug out the T-shirt, and pondered. After a bit more rifling through the refashioning bags, I found a $3 thrifted bargain - a huge, soft man's shirt, new, with tags.
I chopped off the sleeves off both shirts, and trimmed the grey ones to flatten the sleevehead more like the ones from the blue T. I overlocked them on, and did a topstitch around the seam.
Then I cut away the neck, and used the hem of the man's shirt to make a contrast edge, again overlocking on and topstitching.
It's still a sloppy shape, but will be a cosy winter top that I can throw on with jeans! More on the blog.