After some work with the ICAO database people I have secured an official code for the Twister.
When filing a flight plan from now on use the code: TWST
Previously you would have had to use the code: ZZZZ and put TYP/SILENCE TWISTER in the Extra Information section of a flight plan.
Hopefully air traffic will be a little less confused by us in the future.
As you can see they should no longer ask how many POB (Persons on Board) as the description shows 1P (one place).
The confirmation email from the ICAO database is below.
Dear Mr. McKee,
I would like to inform you that the ATDSG (Aircraft Type Designator Study Group) has now completed its review of all the material that you have presented and has allocated the following aircraft type designators TWST for the Silence Twister model. This information is now registered into the Aircraft Type Designator (Doc 8643) electronic database and will be sub sequentially published in the next edition of Doc 8643, which is scheduled for April 2017. I would like to personally thank you for bringing this model to our attention so that it could be registered into Doc 8643.
Manufacturer/Model Designator WTC Description
SILENCE Twister TWST L L1P
Air Navigation Planning & Support Officer
International Civil Aviation Organization