In the end, I took a different approach here.
The manufacturer assured me that the extent of the damage is no big deal, and that keeping the damaged section of the steerer was preferable to removing so much of it that the stem extends beyond the top of it. I wanted to deal with the loose splinters somehow, to prevent propagation, but I figured that epoxy (the manufacturer's suggestion) would have a hard time penetrating well enough to fill all the gaps.
So I decided to file/sand away the damaged part of the steerer, since all but a couple of millimetres would be above the stem anyway, and hence not subject to any significant load. This part of the steerer was structurally compromised anyway, so I saw no reason not to just remove it.
This involved removing several layers of laminate on the frontal section of the steerer. To keep the delamination from propagating further down the steerer, I worked on about 3 mm of the steerer at a time, using a tightly secured strap to demarcate the work area and hold the splinters in place below. I kept going until the cracked area shrunk to nothing, then moved down another 3 mm and repeated, until there was no more visible cracking and the surface was smooth.
In the end, the volume of material removed was about 10 mm tall (from the steerer cut down to the bottom of the splinters), 7 mm wide (around the front of the steerer), and 0.5 mm deep. I found that rubbing a small amount of isopropyl alcohol onto the work area and letting it seep into the laminate (before drying up) showed up any remaining cracks that weren't immediately visible. From the photos below you can see contours indicating around 3-4 layers of laminate were removed in this area.
The worked area extends around 2 mm below the top of the stem, meaning there's a slight gap between the stem and the front of the steerer tube there. I plan to fill in the removed volume with J-B Weld epoxy resin when I have the time, to restore the cylindrical profile.
Below are some photos of the work and the result.
Work beginning. You can see the outline of the damage, with the help of some isopropyl alcohol that was applied beforehand.
Filing the steerer down, with the help of a strap to support the delaminated splinters.
The full width of the steerer, below the damaged area, is 28.65 mm.
The width after removing the delaminated material is 28.17 mm.
Here you can kind of see the contours of the layers that were removed.