In my experience, it depends on the departing station. All ex-Singapore are 100% guaranteed, even a diff class on a connecting separate ticket on another carrier. Manila station is another data point interlining my luggage on a separate tickets to UA, TK, TG.. Data point, SQ tickets departing DEL, the agent doesn't care, EWR a simple, "sorry ,no.".. DME doesn't even listen to what I'm saying. Separate tickets on Star Alliance carrier to SQ? Virgin Australia departing NAN, simply said ," my system cannot see your SQ ticket",.. ok, fine... So by theory, its pure luck if your interlining from separate Star Alliance ticket to SQ, guaranteed departing SIN on SQ tickets to Star Alliance, and few stations like Manila.