Could a small CPLD device do the stuff that needs to be in hardware? - I don't know how complicated it needs to be but the Xilinx 95xxXL CPLDs are very cheap.
Remember though that a lot of USB devices like keyboards and mice actually speak both USB and PS/2, and come with a simple pinout adapter (I dont know if the pin mapping from USB to PS2 is standardised for these devices), so you may be able to get away with the much simple PS2 interface, even if the device has a USB plug on it.
The key word being "a" keyboard. What exactly is that supposed to mean? Do you expect to only work with one specific model of keyboard? If so, you wouldn't have gained much over having a dedicated batch of PS/2 or whatever interface keyboard produced for the specific application, so the whole USB stuff would be just a seriously wasteful detour.
If you want to support all more-or-less standard USB keyboards, I suspect you'd find that implementing this gets you halfway to the full-complexity USB host implementation already. Which means restricting yourself to keyboards doesn't save enough work to outweigh the limitations --- why implement a half-cooked USB host and then add another interface for mass storage devices, e.g.?
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