Re: cart before horse
>it ought to be easier to get permanent power to a seldom used switch than to a seldom used bulb
Unfortunately, at least in the UK, permanent power is normally available only at the ceiling fitting - only one side of the supply goes to the switch from where it is routed back to the primary load (lamp). The traditional way "smart" switches would deal with this would be to constantly leak some current, enough to generate a potential difference across a series secondary load in the switch, just not enough to make the bulb light up.
You'd be better off with a smart ceiling rose than a smart switch.