i too have thought about this,ive done cbt twice now as i used to ride 125 on green lanes with my mates on their 400`s,i still had fun and kept up fine,
I saw a guy in a tm125 with l plates on last year,and a cr125.
But the main problem is they are race bikes and your on a limmit of 14.6bhp on 125`s,ie you cant de- restrict a road legal eg dt125 or ride a comp bike.
bhp on these trials bikes are 33 ish so no you cant do this!