Whenever I trot at the moment out hacking I do so in a 2 point seat, so like my xc position but slightly more upright. 2 point seat in trot is harder to maintain than at a canter or gallop and will quickly build up your calves and also core strength. You need to be able to maintain this position in balance for in excess of 5 minutes (the average BE90 course length) before the season starts and preferably in excess of 10 minutes so you know you will not be sitting down on your horse’s back at the end of a demanding XC course when it needs all the help you can give it.
A nice side effect I find as well is by coming out of the saddle your horse has the chance to really swing through its back which will improve its way of going allowing it to strengthen its own muscles throughout it’s back which should in turn improve your dressage. So it may hurt but it should be a win win situation for you and your horse.