Hi,
When seeking a solution to something like this, it's a good idea to let us know what hardware device you have. Larger HX units have more blocks, and paths, to play around with than the Stomp models.
As you said that by using the Tesselator you can get very close, but cannot achieve the fade out.
O.K., have you tried pulling the Tesselator block down on to a second path and then add a volume block behind it which is controlled by an expression pedal? You can then mix that path back into the main signal chain for any additional processing - reverb, etc. It is then up to you to control the fade with the volume pedal.
Hope this helps/makes sense.