It's strange that most motorcycle manufacturers don't include hazard flashers as standard equipment. The Honda ST1100 is no exception. However, adding hazard warning lights is easy and fairly cheap.
The DPST switch (double pole single terminal) can be mounted in a suitably accessible location. My plan is to purchase a second-hand left fairing cover from an ABS model ST1100 and install the push-button switch where the ABS/TCS switch is located. At the same time I can move the auxiliary lighting switch (which is currently on the dashboard underneath the GPS mount) to the same location.
Bear in mind that when working on any bike wiring, that all connections should be soldered and wrapped in heat-shrink tubing or at least electrical tape for maximum longevity.
The flasher unit can be located either in the tail housing (you can run the wiring to the rear indicators) or behind the instrument panel (you can run the wiring to the front indicators).
NOTE: This installation does not use the bike's existing flasher unit, so you don't have to worry that you'll overload it. It needs to be able to handle a load of 4 x 21W; something like the model RFL2H shown at Vehicle Wiring Products should serve the purpose.
Wiring it up is fairly straightforward:
Disconnect the battery.
From a permanent live feed (e.g. the battery itself), a wire runs via a 10A fuse to the pole on the new flasher unit marked either "B" or "X".
From the "L" pole of the flasher unit, run a wire to the DPST switch (you may need to connect this single wire to two terminals).
Now find the orange wire which runs to the front (or back) left-hand-side indicator, and using a sharp knife, carefully expose some of the wire.
From one of the two remaining terminals on the switch run a wire to the orange indicator wire you have just exposed and solder it in position (do NOT use a "scotchlock" connector). Cover the join with electrical tape.
Now find the light blue wire which runs to the front (or back) right-hand side indicator and repeat the procedure, running a wire from the other switch terminal to this blue wire and soldering it in position. Cover the join with electrical tape. Note that no earth wire is required as the indicator lights are already earthed.
Reconnect battery and test.
Once I have installed my hazard warning lights I will post some pictures!