Just out of curiosity, how would using the prefetch queue in this way serve to copy-protect the game? It seems as though it can be copied.

Well more a sort of protection for the executable. Most often you find
exe-packing/protection when there's some copy protection as well, but
maybe for that game they only wanted to avoid people mess around
with the executable.

Where can I get the code for the prefetch queue emulating CPU? I'd like to roll it into a more modern version of DosBox.

I didn't make the sourcecode public. It's nothing complicated, just modified
the normal core to fetch a bunch of opcodes instead of only the next one.
Seems to emulate the prefetch queue quite accurately i think.

Attached a build from the current CVS sources with prefetch queue
emulation, if you really need the sources for some reason, please
send a PM or something.

I'm not really familiar with DOS box, I actuially built a 386 recently to play these old games on. Jetfighter I (on a single floppy) works great on the old machine. If you need any files from the distribution disk, I have it and the manuals. Faster machines, I think, regardless of the DOS box cause runtime errors. That's my experience anyway for what its worth.

I can confirm that Jetfighter (the original release) has no copy protection. When I bought it a while back I copied the disks and installed/played the game from the copies so whatever protection exists is probably to prevent modification of the executable.