Is it possible to use Microchip's ZCD (Zero cross detect) circuitry in an isolated SMPS?
I'd use either a very small 50/60Hz step-down transformer (in the low single digit VA range) as a coupling transformer if space wasn't a major issue, or a Y1 capacitor in the few-nanofarad range as a coupling cap if I was tight on space.  Either way, the relatively light load of the input shouldn't pose an issue, and the coupling device will act to limit the current so you won't need the resistors any longer.