If a 3-phase, 4-wire RCD is connected temporarily to a 1-phase, 2-wire supply (by looping the 3-phase terminals together at input and output) will the device still function to protect against earth faults

You should not have to link the terminals. As long as current in is equal to current out the device should be stable.

So the 3-phase, 4-wire device can be used in a single-phase installation simply by connecting phase and neutral supply wire at the input terminals and the single phase load wires (phase and neutral) at the output terminals?

pretty much.
Think of it as follows, three phase and neutral into the RCD on the outgoing side you have three single phase lighting loads, one per phase. switch two of the loads off so that one phase (say L1) is feeding its load and returning through the neutral. As L2 and L3 are not carrying any load you basicaly have a single phase load supplied through a 4 pole RCD.

Yes it will work as posted above, the RCD does not "know" the difference between single phase use, and three phase use but with only one phase loaded.

However, dependant on which phase on the RCD is used for single phase, then the internal test circuit may not work, giving the false impresion that the device is defective.

The "test" feature connects a resistor between phase on one side of the RCD and neutral on the other, this causes an imbalance and tests the device.
If the manufacturer has connected the test resistor to "L1" and you use "L3", then the test wont work.

Wire it up and try the test feature, if it does not work, then transfer the incoming and outgoing phase conductors to a different phase on the RCD.

It is possible if you use correct connection. You can refer to Switchgear Riddle No.6 from http://electrical-riddles.com for some wrong and correct connections.

I agree with ArthurHall and broadgage. To be honest, your initial idea of commoning L1, L2 and L3 on the supply side of the RCD and L1, L2 and L3 on the load side will also work, as the current sensing coil will still be in balance.

Regards,

Alan.

I have installed MG 4pole time delay RCD and they state that you link the out put of L1 to in put L2 and connect phase to outgoing L2 and same with neutral. this being you use all contact.