The molex gives ~60w on 12v rail but only 5A. Plus you get ~75 from the pcie slot. So ~195w. I would not do it for fear of something melting or worse. Also the amperage will not be enough even if the wattage is.
http://www.playtool.com/pages/psuconnectors/connectors.html#peripheral
For a given voltage, ampere and watt are directly linked: W=V*A. So if the "wattage is enough" on the 12v line, the amperage is fine too.
That said, I too would be worried if someone were to pull a constant 100W @12v (or even more) from a jury-rigged setup as described.
Issue 1: the "Molex" terminals (They are actually originally made by AMP, not Molex
).
The connectors have 1x12V and 2xGND pins. Even doubling them up (so 2x12V + 4xGND) you are still one 12V cable short of what you would find on a PCIe 8pin connector.
In addition, "Molex" connectors are rated for considerably less current than the PCIe connectors (which are Mini-Fit Jr. Connectors, now actually produced by Molex
) - 5A vs 9A per pin - and have a subpar mating reliability.
So for the 12V line we are looking at 2*5A*12V = max 120 Watt and for the GND line 4*5A*12V = 240 Watt
If even a single one of the 12v pins is not mating properly we are down to 60 Watts. If you are pulling 100W at that point you'll melt the connector right off.
Issue 2: the cable itself.
If a single cable - as it seems to be the case with the FlexATX PSU in question - feeds the 12V pins on both "Molex" connectors, the wire has to be of appropriate thickness to handle the combined load.
If the cable is 18AWG the combined load should be below ~10A, thus 120 Watt. If the cable is thinner it has to be further downrated (for example 20AWG = ~6A = 72 Watt).
Issue 3: the connection inside the PSU.
We just don't know how the wires are terminated inside the PSU. Even if the connectors and the wire can handle the load, the termination inside the PSU might not.