I've just planted an asparagus bed so no idea on it's long term viability but there is plenty round here so I'm assuming it will be OK (I'm in Kent). They need lots of water and, yes, it's key that you don't harvest anything for the first two or three years. They have to get properly established before you harvest and even after that you have to be careful how much you cut otherwise they will get weaker year on year. My supplier did say it would be OK to cut a very few spears next year (we planted while dormant early spring this year) but not sure I'll risk it. How you plant is important as well so I'm not sure how easy that would be to do in a tub - you have a ditch with a small ridge down the centre so the crowns are placed on the ridge with the roots dropping down either side and then you cover them up but they'll still in a ditch which helps the watering. As they get older you can earth up for bigger spears, I think. Have a look on some of the specialist asparagus supplier websites because there's a lot on info there on growing the and the soil type they prefer - maybe it is a soil or weather issue for you that wouldn't apply in other areas.
H