I don't know how to use the stopover rule but I am seeing IAD to SIN flights that book JN from IAD to PEK then from PEK to SIN in X for 80k in April 2023. I wouldn't risk getting off at PEK though with an onward ticket. But I'm sure someone with more knowledge could figure out the excursion perk to get an 80k J ticket to Beijing.

Also another example I am seeing in September (9/9) is 80k from IAD to SIN all in J with IAD to LAX in JN, LAX to TPE in I on EVA, and TPE to SIN in I on EVA.