Help! I've fitted '4-way' switching to a central chandelier over a new staircase, and this works fine, with on/off at any one of four locations. However, with all the sheetrock up, joints done and the paint finished, I realise I really need 2 more switches for convenience. Is it possible to add 'wireless' switching to such a circuit?

Not that I've seen unless you convert all switching over to something "virtually wireless" with one master switch. You'd have to bypass all other switches so that only one of the wired switches controls the fixture. As a single-pole master switch, you would then use wireless switches associated with it. X10 has plenty of gadgets to handle these situations, but I'm not sure if they offer the same flexibility for 230 volt systems.

In 120V land, Insteon will do that--you can use normal Insteon controls in the four locations that you've already wired, and install wireless switches in the other two spots. It should be pretty much seamless.

Also, if you can get wires from any legal circuit to the to two missing spots, standard wired Insteon controls could be used there to control your "6-way" circuit.

I don't think they have a 230V version of Insteon, though.

Is the light in accessible ceiling space? X10 and a small control transformer, controlling a contactor with a 120V coil and 250V rated contacts.... (I know.... it's getting a lil' elaborate )

Phew...
One wireless switch is certainly possible and the easiest solution, though I'm not absolutely sure whether it's the most reliable one. I prefer to keep things wired whereever it's possible...

Other than that... latching relay with 12V coil and push buttons wired with phone or door bell wire...

If you're looking into bus systems there's always EIB... but that's high tech, most likely EXPENSIVE and you need a laptop + the EIB software to program everything. I also have to admit I never heard of wireless EIB switch actors... but that might work.
Personally I call it "European Idiot Bus" 'cause it's so complex to install and program...

Basically You have sensors and actors, both of which connect to the same bus. Sensors can be push buttons, thermostats, motion detectors, you name it. They're all bus powered. Actors are switches or dimmers that switch the 230V power.
Everything is connected to one bus cable and each device gets its own address. During programming you have to tell the system "Sensor 001 is used to switch channel 2 of actor 010" for example.