My team mate is keen to do a bit of drifting in the GTA when it done and were therefore considering fitting a fly off handbrake in adition to the standard cable operated one, MOT implications?

Shouldn't think there will be a problem as the mechanical one is still there.

As Scootz says. A Friend of ine rallies a Mini (proper one) and has a hydraulic fly off one
but the cables are still there. just adjusted so they lag behind the fly off. Passes every
year.

I have no idea how well it works in real life but I remember tiff nedel used a bit of electrical tape on a mini cooper s hand brake to give this effect.

Edit: link to video about 2 mins in http://www.youtube.com/watch?v=HUF8jWqmAB8&feature=youtube_gdata_player

[Edited on 20/3/12 by theduck]

When I was rallying and autotesting, I used to alter the mechanism for the knob on the end of the handbrake so that in normal use it was a flyoff, but if you pressed the button the ratchet mechanism was used.