How to map IDictionary<string, object> in Fluent NHibernate?

Question

I am looking to persist user preferences into a collection of name value pairs, where the value may be an int, bool, or string.

There are a few ways to skin this cat, but the most convenient method I can think of is something like this:

public class User
{
    public virtual IDictionary<string, object> Preferences { get; set; }
}

with its usage as:

user.Preferences["preference1"] = "some value";
user.Preferences["preference2"] = 10;
user.Preferences["preference3"] = true;

var pref = (int)user.Preferences["preference2"];

I'm not sure how to map this in Fluent NHibernate, though I do think it is possible.

Generally, you would map a simpler Dictionary<string, string> as:

HasMany(x => x.Preferences)
    .Table("Preferences")
    .AsMap("preferenceName")
    .Element("preferenceValue");

But with a type of 'object', NHibernate doesn't know how to deal with it. I imagine a custom UserType could be created that breaks an 'object' down to a string representing its Type and a string representing the value. We would have a table that looks kind of like this:

Table Preferences
    userId (int)
    preferenceName (varchar)
    preferenceValue (varchar)
    preferenceValueType (varchar)

and the hibernate mapping would like this:

<map name="Preferences" table="Preferences"> 
  <key column="userId"></key> 
  <index column="preferenceName" type="String" />
  <element type="ObjectAsStringUserType, Assembly">
    <column name="preferenceValue" /> 
    <column name="preferenceValueType"/> 
  </element> 
</map> 

I'm not sure how you would map this in Fluent NHibernate.

Maybe there's a better way to do this, or maybe I should just suck it up and use IDictionary<string, string>. Any ideas?