I've been looking into 'two wire' smart switches which might be compatible with LED lighting, and came across Yoswit - current performance/availability unknown. However, in their documentation they suggest that an LED load of around 5W is the minimum which can provide sufficient leakage current to power the electronics of the smart switch in the 'off' state.
This leakage will be either through the lamp itself (potentially causing a slow ticking as the internal capacitors charge up enough to trigger the LED), through discharge resistors in the lamp's power supply (potentially exceeding their thermal design), or through a parallel 'dropper/leakage' capacitor.
Yoswit claim a standby power for their switch of <1W, but this is still significant compared with a single LED bulb. An array of several LED bulbs is more likely to provide enough leakage to power the switch without illuminating the bulbs.