In theory, yes. There are several external USB and/or parallel port devices offered that use the keyboard port for their power source, especially higher-current USB2.0 devices when used on a pre-USB2.0 PC. For example, I have an older parallel port Camera-Mate card reader that uses this power scheme.
However, you should be aware that this scheme also has its drawbacks. I have seen some systems that would not start with the device connected, but would work normally if the device were connected after startup. It should also be recognized that the actual current available at the keyboard port (do not use the mouse port for this unless there is no mouse connected) will depend upon the designs of the mainboard and the power supply. If both of those components are at the low end of their tolerance ranges, that particular combination may not provide adequate current.