I would say, that this has solution. We have to use a bit more complex SQL in fact. This approach I've already deeply explained here:

• Query on HasMany reference

So, below is just a draft based on your subqery draft. What we are doing, is creating two subselects in fact (check the intended SQL here)

PlanVersion planVersion = null;

// the most INNER SELECT
var maxSubquery = QueryOver.Of<PlanVersion>()
   .SelectList(l => l
    .SelectGroup(item => item.ParentPlan.Id)
    .SelectMax(item => item.ActiveFromDate)
    )
    // WHERE Clause
   .Where(item => item.ParentPlan.Id == planVersion.ParentPlan.Id)
   // HAVING Clause
   .Where(Restrictions.EqProperty(
      Projections.Max<PlanVersion>(item => item.ActiveFromDate),
      Projections.Property(() => planVersion.ActiveFromDate)
    ));

// the middle SELECT
var successSubquery = QueryOver.Of<PlanVersion>(() => planVersion )
    // the Plan ID
    .Select(pv => pv.ParentPlan.Id)
    .WithSubquery
    .WhereExists(maxSubquery)
    ;

having this subqueries, we can ask for plan itself:

// the most outer SELECT
var query = session.QueryOver<Plan>()
    .WithSubquery
    .WhereProperty(p => p.Id)
    .In(successSubquery)
    .List<Plan>();

There could be some minor typos, but the draft should give you clear answer how to...