i was also competing in my 2015 built Megabusa that is non road legal, for which I have a MSA vehicle passport (which clarifies these details) and was competing in Class 3H - Modified Kit, Replica & Spaceframe Car (motorcycle derived engines).
Not having been asked/challenged ever before on this I did ask for clarification as to the ruling in the Blue Book, following the post event scrutineering session. This was provided by another scrutineer:-
"5.16.7. Exhaust catalytic converters must be fitted to all petrol engined production based saloon, touring and sports cars, including specialist production and kit cars, manufactured after 31/12/99. They may be specified for certain other formulae. Competitors are reminded of their obligation to maintain such equipment on a vehicle used on the highway where government legislation requires it"
As mentioned earlier in this post the SSOT are pulling the facts together so they can take it up on our behalf.