I really don't think you're going to have much luck doing that.

USB is a VERY complex protocol operating at 12 Megabits/sec. (full speed)
That's waaaayyyy to fast for those poor little comparators.
And also waaaayyyy to fast to decode with software even if the comparator could handle it.

Plus, there are control transfers and polling every 1ms, and just because you see data on those lines, doesn't mean it either came from, or is intended for the keyboard. It could very well be a control transfer for another device on the bus.

I know you don't want Host interface. But you'll have to, for a USB keyboard.
There are many different dedicated Host chips, like ....

USBwiz
http://www.ghielectronics.com/details.php?id=5&sid=6