The funny thing is that the schematic shows actual HOLD, START and SELECT keys. Incidentally, it also has LSHOULDER and RSHOULDER, but those are available only as test pads (which means someone with enough time could theoretically add them).
So I've though this simple approach:
- POWER + UP = LSHOULDER
- POWER + DOWN = SELECT
- POWER + Y (triangle) = RSHOULDER
- POWER + B = START