EF1: Filtering derived types of entity class using .OfType<> by passing a string value

I have a situation where I'm trying to filter a LINQ select using a derived sub class.

ctx.BaseEntity.OfType<SubClass>() - this works fine.

However I'd like to do this using a string value instead. I've come across a performance barrier when I have lots (>20) Sub Classes and selecting an Entity without using OfType just isn't an option. I have a generic UI that renders from the base class, so I don't know what Class Type will be returned at compile time.

So what I'd like to do is this:

1. Perform a projected Select where I return just the SubClassType from the database

2. Perform a second select using this value as the OfType to only select the relevant related entity from the database (No mass unions generated)

       int id = 1;
       var classType = (from c in ctx.BaseClass.Include("ClassType")
                                  where c.id == id
                                  select new
                                             {
                                                 c.ClassType.TypeName
                                             }).First();
   
       BaseClass caseQuery = ctx.BaseClass.OfType<classType.TypeName>()
                       .Include("ClassType")
                       .Include("ChildEntity1")
                       .Include("ChildEntity2")
                       .Where(x => x.id== id);
   

But obviously this won't work because OfType requires a Type and not a string.

Any ideas on how I can achieve this?

Update: As a side note to the original question, it turns out that the moment you project a query that uses a Navigation Property - it builds the monster SQL too, so I've ended up using a stored procedure to populate my ClassType entity from the BaseClass Id.

So I've just got it to work using eSQL, which I'd never used before. I've posted the code here just in case it helps someone. Has anyone else got a more strongly typed solution they can think of?

BaseClass caseQuery = ctx.BaseClass.CreateQuery<BaseClass>("SELECT VALUE c FROM OFTYPE(Entities.[BaseClass],namespace.[" + classType.TypeName + "])  as c")
                .Include("ClassType")
                .Include("ChildEntity1")
                .Include("ChildEntity2")
                .Where(x => x.id== id).FirstOrDefault();