Managed to borrow an appropriate IDE drive bay, plug it into an open USB port on my hub, and extract the data from the hard drive using
Note that the whole drive is being copied, not just a single partition.
dd if=/dev/sde of=~/macHD.dd
The resulting image file can then be mounted under BasiliskII like any other drive image, and any file conversion (etc.) can be done using (68k) Mac programs from within the emulated environment. Converted files can then be copied to the Linux filesystem via the "Unix" virtual drive on the emulated Mac desktop.
Alternately, the resulting image file can be burned to a DVD (in my case, it was small enough, since this is a really old hard drive) as an image file, then mounted under BasiliskII by simply putting it into the DVD-ROM drive.