Very creative and interesting ie 'use whatever tools/knowledge you have' but:
> The internal hard drive uses SCSI with an unusual connector.
This is literally never an issue as old connectors are easily obtainable (in particular 'in this day and age').
> Adapting it didn't seem straightforward, and we weren't confident the old file system (HFS) would be easy to read from a modern system.
Simple search pulled this up. Honestly that reason didn't make any sense to me at all. The other way might be interesting and fun but I don't think this is a reason not to attempt mounting the old file system.
Hell, I wrote a FUSE implementation of HFS a while back. HFS (and to a lesser extent HFS+) is quite well documented. Current versions of MacOS can still do HFS+ (but not HFS) natively.
Fun for the win! I suspect most of the other suggestions, even if possible, wouldn't have generated this level of interest. Well done on your Rube Goldberg method!
Linux will mount HFS drivers and files like nothing...
This setup it's overcomplicated and hackey, but dumb in the end. You can mount old SCSI drivers with modern adaptors just fine, and Linux/BSD will either mount or extract HFS images with unar (I think) or hfsutils/hfstools.
Or just spawn ymodem/zmodem compatible software (as he used a Fax setup for modem) and the problem would be solved in minutes.
I sympathize with the notion to fix things in software instead of buying hardware (an SCSI adapter): You have to spend money, wait for it to arrive (or even leave the house to get it yourself!) and it's one more future plastic waste.
If fiddling around and getting creative is something one enjoys (and has the time to do so), doing things like OP did is a nice alternative :)
> The internal hard drive uses SCSI with an unusual connector.
This is literally never an issue as old connectors are easily obtainable (in particular 'in this day and age').
> Adapting it didn't seem straightforward, and we weren't confident the old file system (HFS) would be easy to read from a modern system.
Simple search pulled this up. Honestly that reason didn't make any sense to me at all. The other way might be interesting and fun but I don't think this is a reason not to attempt mounting the old file system.
https://www.matthewhughes.co.uk/how-to-mount-hfs-classic-dri...