Some Dell systems include only USB ports by default, making it difficult to use a PS/2 keyboard or mouse for low-level OS hacking. It turns out that, for the Optiplex 755, there is a Dell product that adds PS/2 keyboard / mouse ports and an additional serial port to the system. However, the European and US Dell accessories pages list the item differently. The European item is shown to be compatible with the Optiplex 755, while the US item is not.
I took a leap of faith and ordered the US version anyways. It is now here, and it works perfectly.
Unfortunately, the Optiplex 755 User’s Guide does not mention the expansion connector on the motherboard that is required for this device. My regular desktop-sized Optiplex 755 included the necessary connector, and everything worked out of the box!