I recently contacted VOSA about this after my MOT, and got this reply:
"Dear Sir/Madam,
Thank you for your e-mail enquiry dated 30th March 2010, concerning MOT emissions.
When testing the emissions of a vehicle for the MOT test, the MOT tester should check their "In Service Exhaust Emission Standards for Road Vehicles" book to check if there is an exact match for the vehicle they are testing and it's engine code.
If there is an exact match, then they would test the vehicle according to the emissions levels stated for it in the book. If there is no exact match then the vehicle would be tested according to it's age.
Passenger cars that have no exact match in the book and are first used between 1 August 1986 and 31st July 1995 would require a non cat test and would need to meet the following limits: CO
You can view the latest In Service Exhaust Emissions standards for road vehicles via the link below (click on section 1 or section 2 CSV).
http://www.dft.gov.uk/vosa/inserviceexhaustemissionsstandardsforroadvehicles.htm
You can also view the MOT inspection manuals via the link below:-
http://www.transportoffice.gov.uk/crt/doitonline/bl/mottestingmanualsandguides/mottestingmanualsandguides.htm
There is not an exact match in the emissions book for a 1993 Toyota Supra RZ, so it should be tested according to it's date of first use. "
So basically only a non-cat test is needed on older Supras. I'll be printing this out and taking it with me next year!