> I have just noticed that the machine hangs at the NetBSD boot prompt
> when the serial cable isn't plugged in

Perhaps it is waiting for carrier detect?

> it just refuses to actually do the timeout countdown and boot the kernel
>  until I plug it in, hit enter, which brings me a prompt, and type 'boot'.

Does it autoboot if the cable is plugged in?  Or do you always have
to type at it?

> I can't have the cable plugged in to my laptop 24/7,
> as my laptop happens to sit on the other side of the room.

Use a longer cable?