Alternatively, if there was a way of closing the layby at night that might help as well. A lot of the drunks and eedjits might be discouraged by a two hundred yard walk.
Yes, that is a good idea. That's a busy road, from memory, and anyone carrying anything nasty might think twice about it if they could be seen by passing motorists. The Trust might suggest it to the Council (if it's the Council's responsibility) and argue the case on the grounds of helping to preventing vandalism. 