@aaronm well, pfft! Tobias Johansson kindly chatted with me and provided an spk to test out the LEDs, which worked, but using the source he provided, which wasn't significantly different from mine, I still couldn't get my LEDs to blink. After a pause to work on other things, I came back to this and turned various SDK configurations on and off - one at a time so I knew what I changed - and found that turning off the ARCH_LEDS setting enabled my program to control the on-board LEDs. I probably turned it on at first thinking it was related to LEDs and therefore necessary.
Many Thanks! to Tobias.