Do I understand this correctly.... the Pro Mini can quite happily be programmed from any old serial port just by pressing the reset button, and releasing it just as the IDE finishes compiling and starts uploading- there seems to be a window of about a second... or have I badly missed something?
I think sort of. However if you can connect DTR that resets it for you when required.
It has to be a TTL (0-5V) serial port. An RS232 port (like the 9-pin connector on a PC) has the wrong voltages and polarity.
I find that releasing the Reset button when the RX LED blinks is a good way to get the timing right, for those boards that have LEDs for the serial pins. For other boards the message "Binary sketch size:" in the sketch window indicates a good time to release the Reset.