Well, if the bare.i kernel from Slack 8.1 was working with a USB keyboard without USB modules loaded then I suspect it was using BIOS compatible mode. By enabling the USB ports and making them hot it would disable the BIOS function.
After loading the usb-uhci module, then also load the usbkbd, input, and keybdev modules for the keyboard support. Of course you know by now that you can't type these commands in to the system. They have to be loaded from a script.
You can create a script for the above and run it manually. You will need to do a "chmod 755 script_name" to make it executable after you create it. Use pico editor or other if you like. Then run it by using "./script_name". If it works OK then copy it to the /etc/rc.d directory and call it from the /etc/rc.d/rc.local script if you like. But I think they are also called from the rc.modules script if you locate them all and remove the remark from the line to enable it during bootup. The exception from what I could see is the "input" module, but that would autoload from the dependicies I would think for keybdev. But be carefull because the rc.modules executes on each boot. If it doesn't work then you will not be able to even login to the system to make any changes to it because the keyboard will not work. That is why I suggested using a separate script to make sure it was working prior to making the change to the startup scripts. The worse case there is to reset the system or maybe use the mouse to shutdown if GUI operation.