And Claude, helped quite a bit. Got it working!

The essential bits I'm calling in my c++ node are:

void setExternalData(const ExternalData& d, int index) override
   {
       SNEX_INIT_FILTER(d, index);
   }

   // HISE calls this (message thread, on repaint) for each frequency point.
   // freqNorm is normalised to the sample rate (1.0 == fs); return linear gain.
   double getPlotValue(int /*getMagnitude*/, double freqNorm) override
   {
       return (double) magnitudeLinear((float)(freqNorm * fs));
   }

   // == Parameters ==========================================================
   template <int P>
   void setParameter(double v)
   {
       if      constexpr (P == 0) { pFreq   = (float)v; sendCoefficientUpdateMessage(); }
       else if constexpr (P == 1) { pReso   = (float)v; sendCoefficientUpdateMessage(); }
       else if constexpr (P == 2) { pGain   = (float)v; sendCoefficientUpdateMessage(); }
       else if constexpr (P == 3) { pDrive  = (float)v; sendCoefficientUpdateMessage(); }
       else if constexpr (P == 4) { pType   = (float)v; sendCoefficientUpdateMessage(); }
       else if constexpr (P == 5) { pMode   = (float)v; sendCoefficientUpdateMessage(); }
   }