One of the ps/2 to usb converters I ordered has arrived! It works fine on a Pi 2 and on Windows, but it seems to not get enough power off the Zero. Connecting everything to a powered hub seemed to solve this, but if I'm wanting this to work on a battery pack, this could be a problem.
I do have another converter on order, one that's a bit newer, and not built into a hub. We'll see how it goes when that gets here. Also on order is another Ethernet dongle, and a few other parts that will be useful... like a micro sd extension! This means I can have the micro sd accessible without cracking the case open every time, and will let me position the pi in the best place in the case, probably based on where I want the hdmi port to be located.