How to implement custom SQL statements for entity's property in NHibernate

In application I have a bunch of entity class NHibernate mappings with two common properties: {GUID Id, string Tag} (it is implemented through abstract base class).

In DB there are tables for each entity with common columns:

d开发者_如何学Cbo.[EntityName] { uniqueidentifier Id,  ... etc. }

Also there is a table that stores information about some global tag for each entity:

dbo.EntityTag { uniqueidentifier Id,  nvarchar Tag }

This table is strictly needed and it's mandatory to have it in Db.

How could I implement custom SQL logic to update, select and insert values in Tag property?

For example in FluentNhibernate It would be helpful to have something like

Map(x => x.Tag)
    .Insert("INSERT dbo.EntityTag VALUES({Id},{Tag})")
    .Update("UPDATE dbo.EntityTag SET Tag={Tag} WHERE Id = {ID}")
    .SELECT("...")
    .Delete("... etc")

Formally speaking, you can do it using custom implementation of entity persister.

However, in this case, it seems easier to use a <join> mapping.