Eagle has a symbol for an old-style 8-bit ISA edge connector. It's called IBM8BIT, and it's in the con-pc library. Unfortunately, it has some extra gunk on it (dimension lines, a PC board outline, and the outline of that metal thing where external ports go) that I just didn't need when adding an edge connector to the ALU board.
As my second adventure into Eagle library editing, I copied the symbol from the con-pc library and removed the gunk. The newly-slimmed-down library can be found here.