Ok here's the solution - well it doesn't answer my question but it gets the job done.
const var Sampler1 = Synth.getSampler("Sampler1");
const th = Engine.createTransportHandler();
th.setOnTempoChange(true, updateTimeStretchRatio);
// fetch the current state
const var obj = Sampler1.getTimestretchOptions();
// Apply your changes
obj.SkipLatency = true;
// Update the options
Sampler1.setTimestretchOptions(obj);
inline function updateTimeStretchRatio(newTempo)
{
local ratio = newTempo / 140;
Sampler1.setTimestretchRatio(ratio);
}
