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.

• Diode level shifter (description, circuit): did not boot

This article has several ideas for interfacing: Exploring Edison - Life At 1.8V. From that article:
• Single transistor non-inverting (circuit): did not boot
• Two transistor, non-inverting (circuit): worked!

MOSFET level-shifter (description). I found two commercial implementations:
4-channel I2C-safe Bi-directional Logic Level Converter - BSS138
SMAKN® 3.3V To 5V 5V To 3.3V 4 Channel IIC I2C Logic Level Converter
But neither of these worked.

Finally, I tried a chip designed for this purpose, a 74AHCT125. I used a DIP version to breadboard: 74AHCT125 - Quad Level-Shifter

That worked, so following this post, I soldered a 74AHCT2G125DP,125 (SOT8) onto an SMT Breakout PCB. That was easy to wire into a DotStar connector.

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