Hello Adam ,
Unfortunately the types varbinary and longvarbinary are not yet supported by Telerik OpenAccess ORM to be directly passed in the GetSqlMethod (note that to execute a stored procedure explicitly you must relay on the GetSqlMethod). The only workaround would be if it is possible for you to use the stored procedure implicitly. Internally we are doing some conversions that will suppress the problem until the problem you encountered is fixed.
the Telerik team