I tried a number of solutions. Here’s what worked and what didn’t. “Worked” here means that the Omega2 booted correctly and then successfully drove the DotStar LED strip via the SPI interface using Python code linked above.
I found that I had to connect the output enable pins of the 74AHCT2G125 to ground. That surprised me, as I expected that they would have to driven by the Omega2 SPI CS1 pin, allowing the Omega2 clock and data pins to float while the Omega2 was booting. But with the OE lines soldered low, it worked, booting and then driving the LEDs via SPI.
I soldered the pins of a mini-expansion board as described in the original post above, wiring the +5V from the DotStar power supply, via the connector cable to the 5V pin of the USB A socket on the mini expansion board. It's large enough to be probed/soldered to. [Note: With this setup, power is now supplied by an external supply. Do not connect to a laptop using either USB port.]
I sent this question to Onion tech support, but received only a note that it would be forwarded to technical staff.
it looks like the Omega2 headers are labeled "A" and "B" and numbered left to right. Diagram here
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