I need a break from this so here are my findings do far. Maybe one of you will have any idea if there is anything else to try.
I think I have identified most of the pins in psu board connector. They are marked in the picture attached below.
Identifying ground, 3.3V and 5V rails was pretty straightforward - just look up few ICs datasheets. There does not seem to be any local regulation on the main board, apart from isolated IO section, which has its own LM7805.
I had more problem with tracing 24V needed for relays operation. I was not able to identify TSSOP-16 driver IC but one of the pins was directly connected to protection diodes on the cathode side. This also connects directly to one of the pins on the external module header so I am pretty sure that this is indeed 24V rail.
Two pins in the top row do not seem to be connected anywhere except for the screw header. On my unit (pn: 1763-L16DWD) those places are marked as "not used" but on the AC powered versions of this PLC they are used for providing 24V dc for sensors.
However, after connecting power all I am getting are two LED lights: "Power" and "Fault". In the manual this condition's probable cause is described as "Loose Wiring" or "Processor Hardware Error". Normally I would just give up at this point but after reading some forums it seems that this state is often caused by power issues so I believe that main board may still be ok. (numerous) clocks are present and LCD seems to be initialized a second or so after power up (faint pattern appears) but no activity besides that. Like some brownout protection was holding device in reset.
I guess it might be related to the one pin in P2 Header I was not able to identify - marked in red in annotated photo. Its location suggests that it may be separate 24V ground but after probing with multimeter it seems that it is only connected to one of the pins of Rockwell branded Atmel part on the main board (I suspect it's either ASIC or AT40K fpga. It has 64MHz clock on one of the legs and in a bigger PLC model in the line - ML1400 there is an Altera cyclone next to Coldfire MCU also present here) so it seems to be logic IO of some sort. I have two ideas for what this line might be for: "power on" signal from MCU to PSU, allowing for some kind of power sequencing or some sort of "power good" signal form PSU to MCU keeping it from booting until voltages are stable. This pin seems to be constantly in high impedance but applying 0V or 5V to it do not change device behavior.
I'd be interested if you have any ideas how to bring it back to life. Of course, one of the ICs on main board may be fried in which case there is not a lot that can be done but playing with it was still fun.