I can only give you ideas on part of the problem you are trying to
solve.  Several people have written routines to allow one to put a PIC
in the electrical path between the keyboard and the P.C.  The PIC can then
exercise the keyboard data lines to fake key presses.  Another factor you will
need to think of when trying to stuff the keyboard buffer is that there are
two characters that get stuffed.  One is a byte called a scan code and the
other, supplied by the operating system, is the character.  Different
keyboards such as the 83-key type or the AT-style keyboard generate totally
different scan codes for the same keys so you will have to design your
system for the right style keyboard or even several styles.  You could
still use a com port for the letter preview function, but the actual sending
process to the buffer is probably easier if you can actually fake key strokes
to the keyboard input port.

Martin McCormick