Well, it worked!
I had to make a boot disk that assigns the SYS: to the hard disk, but the old girl (A1000) seems not to mind the A590, and runs fine.
Also, I was amazed to see the date was still accurate after all this time sitting packed.
Only thing that went wrong was the A1000's mouse with the angled plug. It is no longer working. I opened the mouse itself and found a fair amount of corrosion had built up inside the circuit board. I tried to clean it, and now I get vertical pointer movement and the buttons work, but no horizontal movement.
Pain in the arse, as the A500 mouse will not fit in the joyport, because the A590 housing is now in the way.
A small detail though. Oh yes, the 2 MB of Ram in the A590 is working as well.