Martin,
The current needed by rocker switch lamps could be as high as 50mA at 3V. It is conceivable you could extract it from the filament winding via a single diode, black band (cathode) to the lamp, with the other terminal of the lamp returning to the other filament winding.
But you will need a resistor, of around 68R 1W, after the diode to drop the voltage from the 6.3Vac down to a suitable DC level. No smoothing cap is required, as this is an incandescent lamp of high thermal inertia and pulsing won't be evident.
This sort of current is not obtainable from the HT circuits, unfortunately.
Cheers,
Hugh