R – NHibernate: Lazyload single property

I'm a bit confused with your problem. I feel like following Hibernate best practices should be enough, no need to re-invent what already there. Here are a few comments.

The idea that I have to maintain some sort of reference to the ISession and make sure the entities are associated with the session before accessing a property that might trigger lazy-loading is a bit more plumbing in my viewmodels than I appreciate

If you follow the open session in view pattern, this becomes really easy.

Where the idea is that the ormService will simply disregard the lazy load request from the entity if the collection has already been loaded

That's what the lazy proxy does already.

Basically, i need a way of telling nhibernate someting like: "hey, populate this property using this session which i'm giving you".

That what you do when you detach/attach objects to a session. The lazy items will be loaded according to the session the object is attached to.

You have to think of your QueryOver query as (nearly) directly translating into SQL. With this in mind, imagine this SQL query:

select
    Album.*
from
    Album
where
    Album.Name = 'SomeAlbumName' and
    Album.Artist.Name = 'SomeArtistName'

This won't work because you can't access a related table's properties like that in a SQL statement. You need to create a join from Album to Artist and then use a Where clause:

var riAlbum = 
    session.QueryOver<Album>()
               .Where(al => al.Name == albumName)
           .JoinQueryOver(al => al.Artist)
               .Where(ar => ar.Name == artistName)
           .List()
           .FirstOrDefault();

Also, since you're using FirstOrDefault, you may want to consider moving that logic to the database end. Currently, you're pulling back every record matching your criteria and then taking the first one. You could use .Take to limit the query to 1 result:

var riAlbum = 
    session.QueryOver<Album>()
               .Where(al => al.Name == albumName)
           .JoinQueryOver(al => al.Artist)
               .Where(ar => ar.Name == artistName)
           .Take(1)
           .SingleOrDefault<Album>();