I've been travelling (to VCF-East) and so have missed the start of this thread. But in general...
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The Z80MC CPU board can be run all by itself. It's all CMOS, and draws so little current that I can run it on just the power from my RS-232 adapter. It has diodes from the two PC handshake lines and serial output to a filter capacitor. When my modem program sets the handshake lines high, it delivers 10-15mA, which is more than enough to power the 5v regulator and board.
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You can use a NV RAM, but they generally write-protect themselves before the Z80 and related circuitry crashes from falling power. Thus whatever your program was doing when the power fails will crash.
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As Allison noted, you can use NMI to warn the Z80 to save its state. You'd need a little circuitry to detect when the input to the 5v regulator is getting low, and pull the NMI pin low. Write an NMI interrupt handler to save the state in your NV RAM. It could be an old 60 Hz transformer-rectifier power supply that normally delivers about 8-10vdc. When it falls below about 6vdc, it pulls the NMI pin low.
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The Front Panel uses the most power when the LEDs are active. If the Z80 stops handling interrupts, it times out and blanks the display; so standby power is pretty low.
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When I can find them, I provide 16C450's for the SIO card. It's CMOS, and so draws a lot less power than the 8250. (The 16C550 won't work.) If you have an SD-card, the NMI handler could save the state on the SD-card.
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Since total power consumption is pretty low, the simplest scheme might be to simply power everything from a modest size battery.
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Lee
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Excellence does not require perfection. -- Henry James
But it *does* require attention to detail! -- Lee Hart
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Lee A. Hart
Lee Hart