i would say that you would have to keep the bike as it was at the time of the mot,or get it re-moted after you fit the lights,
it will be interesting to see how this pans out,as i have had a few bikes in the past few years on a daytime mot,never had any problem obtaining the mot,and when insuring them with footman james,was never asked what type of mot i had,as stated before trials bikes have to use the road,and i'm in the same boat as you,as i'm to building a spitfire style bike,and a goldstar scrambler,without lights,so i guess in the next OBM they will shed some light.