It can be done in a day if you get up early enough to catch one of the first trains of the day to London. We stayed in London and caught a breakfast time Eurostar from St Pancras to Brussels, then Thalys to Cologne and DB to Hamburg arriving in time for an evening meal.
Probably no good for most business travellers but a pleasant way if you're not in a hurry as a leisure traveller like us. Didn't save much if any money over bmir but the train was more enjoyable and better value - to us anyway.