I have now found out that I could use the Take() extension method on the IQueryOver instance, and only the enumerate to a list, like so:
Result precedingOrMatchingResult = Session.QueryOver<Result>().
Where(r => r.TimeStamp < timeStamp).
OrderBy(r => r.TimeStamp).Desc.
Take(1).List(). //enumerate only on element of the sequence!
FirstOrDefault(); //get the preceding or matching result, if there is any