After realizing that you mentioned all 8 GPIO lines were on the 20-pin expansion header J5 in the microdesktop case, I consulted the microdesktop schematic for clues.
I suspect the UART and EOMA I2C pins should be left to those functions.
I have added tables to the "Testing"[*] page under the "GPIO" section with my nominations for which pins to test and their mapping back to A20 register bits.
Luke, does this match your understanding of the GPIO pins to test?
Reference: [*] http://rhombus-tech.net/allwinner_a10/testing