For the two-LED series variant, one could wire a diode in parallel with each LED and switch, and hide an AC generator int the "battery". Surface-mount diodes attached directly to the bases of the parts might not be readily visible. For the two-LED parallel varient, wire diodes in series with the switches (the LEDs themselves wouldn't need added diodes) and use an AC supply that won't mind being shorted in either or both directions. The three-LED approach could be done with a number of wiring approaches, though I can't think of any that would be as simple as the two-LED approaches. Perhaps the cutest such approach would be to put a small capacitor and inductor in parallel with each LED and with each switch. Each LED would have a different LC pair, and each switch would have an LC pair which matched one LED. Provided that the AC supply produced the three required resonant frequencies, the LC pair on each switch would kill a particular frequency while passing through all others; the LC pair on each LED would bypass the LED for all frequencies except the one which would be killed by the corresponding switch.
