There still no evidence that these 350 on LAX-SIN can operating at full pax capacity though (i.e., no seat blocked, full 253 pax plus luggages). As you said, when cargo is more lucrative, maybe they are trading pax weight for cargo weight.
There is evidence.viewtopic.php?t=1405913&start=60
SQ estimates for using 101.4t of fuel and 10.1t of reserves.The relevant V speeds suggest TOW of ~273t and so a landing weight of 172t.
Knowing that 150pax weigh ~15t then we can deduce that the aircraft without fuel or pax would be approximately 147t.viewtopic.php?t=1447519
in this thread in reply 7 you can see a representative A350 with a DOW of ~135t
This suggests that the SQ flight was also taking ~10t of additional payload.
In any case we can see that the Breguet range factor for the aforementioned 8285nm flight ((UL/D)/TSFC)) would be ~17933 .....(Mission_Range/(LN(TOW/LW)).
If we apply this same range factor to a 7621nm flight (LAX-SIN) then we can see that with an MTOW takeoff we would achieve a landing weight of 183t.
If we take away 10t of reserve fuel (assuming that SQ are this conservative on all flights, not just the first time operating the worlds longest flight in some years) we get 173t. If we then remove the 25.3t of pax weight (100kg each)we then have a weight without pax of 147.7t, this leaves plenty of room for cargo.
Lets build it up the other way however.
Assume there is full pax (253) added to the aircraft along with 5t of reserves and 5t of pantry (135.5+5+5+25.3) gives 170.8t. Assuming it takes off at MTOW (280t) then using the same Breguet Range factor it will have an SAR of 8864nm. For this aircraft to be in jeopardy of not making it with a full pax load it then has to encounter average headwinds in the order of 68kts...
If by evidence you mean you want to see a load sheet then I'm afraid I am of the same opinion regarding there being no evidence to suggest that the B787-10 can take a full pax load from LHR to MCO....