hey there, (this is not elles btw)
i have not actually trialled, but i am going to next year so i have gone and found some stuff out.
"is it possible for freshers (i.e. people beginning at Oxford in October 2006) to trial for the lightweight boats?"
yes it is possible, it would be advisable to get in contact with the club and possibly also your college as the training starts before term does officially, so you'll need somewhere to stay.
Secondly, what sort of 2K erg score would you need? (Again, the OULRC site says anything under 6:50, but I was wondering if in reality people are much faster than this.)
obviously below 6:50 is good, but really they dont care what your erg score is as long as it is ok. The training will make you a lot fitter and stronger so dont worry on this score. I know someone who trialled and had never got below 7:00, he stroked nephthys last year.
Lastly, would a mere year and a half's experience of rowing on water be enough? At the trialling stage is more importance placed on your erg times or your on-water performance?
AFAIK on the water performance is the only thing that matters, OULRC race cambridge in boats not in the gym. Only a year's worth of experience is fine, plenty of people trial with only a year of college rowing behind them. In some ways it is an advantage as you wont have learnt any bad habits yet.
What exactly happens at trials?
Cant really answer this one, but 12 training sessions per week i think is the norm, and be prepared for it to be very tough.