Atari ST floppy drive?

I can't help you much on the Mega4 ST keyboard issue, having left the Atari universe before the Mega4 came out. However, it seems reasonable to assume that the Mega4 follows the 520 and 1040 in the keyboard design. If this is so, there are several problems. The first is that the serial signal levels are TTL, not RS232C, so a generic serial port interface is out of the question. The second is that the bitrate is a little odd (7812.5 bps) and not one you'll find on a PC serial interface. On the ST, the keyboard processor also handles the mouse and joystick as well as the time-of-day clock. Keypresses are simple key-number depress/key-number+128 release codes, but there are also a couple dozen commands from the CPU that the keyboard understands, so we're not talking about a simple terminal interface here. It might be possible, if you're handy, to take a PC keyboard and a microcontroller and concoct what you require for the Mega4 with little more than a bunch of programming. But I'll leave that exercise to you!