Never mind -- I stumbled across an earlier StackOverflow post that provided the answer I was looking for, shortly after I posted. Here's my original example rewritten using QueryOver syntax:
var disjunction = Restrictions.Disjunction();
foreach (var tuple in OrdersAndLineItems)
{
var conjunction = Restrictions.Conjunction();
var order = tuple.Item1;
var lineitem = tuple.Item2;
conjunction.Add(Restrictions.On<SomeClass>(x => x.OrderId).Equals(order.Id));
conjunction.Add(Restrictions.On<SomeClass>(x => x.LineItemId).Equals(lineitem.Id));
disjunction.Add(conjunction);
}
var result = Session.CreateCriteria<SomeClass>().Add(disjunction).ToList();