GeometryReplacer has an option for "EncodedPolyline". This supports polylines encoded to Google's documented 5 decimal places. But I have a polyline encoded to 6 decimal places, and so had to use PythonCaller and some ugly hacks to get a WKT formatted polyline.
Why am I using 6 decimal places? I'm using the Valhalla open-source routing engine for OpenStreetMap (https://valhalla.github.io/valhalla/).
Valhalla's reasoning for using 6 decimal places is documented at https://valhalla.github.io/valhalla/decoding/
I used the Python script in FME to get my decoded polylines, but would prefer an option in the GeometryReplacer for "Encoded Polyline 6dp" below "Encoded Polyline"
Their demo tool at https://valhalla.github.io/demos/polyline/ is useful in debugging.
Here's a random 6dp encoded polyline:
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