This is a real wildcard option and no idea how the cost would play out (probably quite pricey) but you could go SFO - LAX - NRT - SIN - JNB. SFO - LAX obviously on UA metal and then LAX - NRT - SIN - JNB on SQ metal which all earns 100% on UA if in B, E, M, H, W, U, L, K or higher.
It would maximize the miles but sounds pretty awful if you were stuck in Y for the whole thing (~14500 nm each way per gcmap)