This is awesome!

I'll certainly contribute to this. Will the source be opened up?

Here's a delay filter:

  var delay_array = [];
  var delay_length = 5;
  var delay_feedback = 0.6;
  var delay_volume = 0.4;

  function delay(x) {

    y = 0;
  
    if (delay_array.length < (delay_length * 1000)) {
      delay_array.push(x);  
    } else {
      y = delay_array.shift();
      delay_array.push(x + (y*delay_feedback));  
    }

    return (x) + ( y * delay_volume );
  
  }

Include this in the DSP return code, ie;

  return delay(synth);