T-sql – Percentage difference of the last value from the previous values of a column based on certain data group within the same table

;WITH CTE
     AS (SELECT OrganizationID,
                CustomerID,
                OrderID,
                COUNT(*) OVER (PARTITION BY OrganizationID) AS C,
                ROW_NUMBER() OVER (PARTITION BY OrganizationID 
                                       ORDER BY (SELECT 0))     AS RN
         FROM   YourTable)
SELECT OrganizationID,
       CustomerID,
       OrderID
FROM   CTE
WHERE  RN <= CEILING(0.5 * C) 

SQL Fiddle

I skipped the code example, and jumped to what seems to be the real question here

On top of that, a manual inspection of the index in question shows no text, ntext, image, xml or varchar(MAX), nvarchar(MAX) or varbinary(MAX). Could there be something I'm missing here?

For the record, this is a clustered index.

You certainly are missing something, and the answer is in the body of the error message:

For a clustered index, the column could be any column of the table

For Online Index Operations there is a difference between a Clustered and a Non-Clustered index.
both may not contain a BLOB column in order to rebuild online,
but while a Non-Clustered Index "contains" the columns in it's definition (and included columns),
a Clustered Index "contains" the entire table with all of the columns.

in your case there is a column named MEMO which is of data type text, ntext, image, varchar(max), nvarchar(max), varbinary(max), xml, or large CLR type.
this column is part of the table, and thus part of the Clustered index.

Your options are:

1. Perform the REBUILD operation offline
2. Upgrade to SQL Server 2012
3. Change the column's data type

Guidelines for Performing Online Index Operations