When does that error happen? Or rather do you need to install the game and then when you run it, this error happens? (maybe you didn't keep the floppy(img) mounted)
Maybe you need to set the label correctly for the floppy.
Does it work in real dos?

There is no installer. The game is run from the floppy. Label of the floppy is "CARRIER PC". I don't have a real DOS machine to hand now, but will test it there tomorrow.

ETA: Game runs in pure DOS with no problems.

There is a switch to the mount command to set a label.

It might be that the game uses some copy protection that will require you to boot dos in Dosbox

Using a bootable MS-DOS 6.22 HDD image, I see that the floppy image has the correct label, but running the game gives me the same key disk error.

I understand there is no way of accessing a physical floppy in this environment?

DOSBox mounts only the files on your physical disk. .IMA-format image files only contain standard sector data but no low-level structures. Keydisk checking routines depend on the latter. Keydisk-protected games are invariably incompatible with (vanilla) DOSBox in their unaltered (uncracked) state.

If you can make a floppy image, then I'll suggest making a .TD0 diskimage using Teledisk. Maybe it works with PCE emulator (http://www.hampa.ch/pce/pce-ibmpc.html).