As noted previously, a couple of issues with the Version 0.1 PCB meant a slight bodge had to be made to the board for it to function correctly, and using a 12V supply would reach the absolute maximum voltage at the output MOSFET gate.

Fortunately, it appeared the 'missing pulse detector' part of the circuit worked as expected, so it should just be a matter of updating the output stage.

First and foremost, the output MOSFET Q1 has had its drain and source pins switched, so that the body diode acts to block current flowing from VCC (rather than conducting). Additionally, the pull-up and gate resistors have been changed to form a potential divider, so that when the control signal is pulled low, the gate-source voltage reaches ~-10V at VCC=12V, giving a couple of volts of margin from the absolute maximum rating. 

I had considered simply using a MOSFET with a larger gate-source voltage limit, as the divider solution would also reduce the drive voltage to ~4.2V when operating from a 5V supply. However, when looking at alternative transistors, most datasheets for common, cheap devices indicated higher on-resistances than the A03401A claimed, even with the reduced gate voltage. Since rearranging the resistors costs essentially nothing, I opted to keep the AO3401A using the potential divider, and just accept the slight (probably unnoticable) increase in on-resistance.

Changes to the PCB layout were relatively small. Q1 was rotated 180 degrees since this lined up better with the power traces, and a few signals were shifted around to the updated resistor pads. Finally, some small updates to the silkscreen were made to tidy up the component references, and to help tell them apart from the version 0.1 boards.

Compared to the version 0.1 layout, the trace routing actually looks slightly less convoluted in my opinion, but perhaps I'm just telling myself that to justify having to pay for a second batch of prototypes....