Firstly the WCF SQL adapter. While the hashkey table idea may provide some level of comfort it will kill any scalability. You are basically single threading the the database access. Only the first host instance will ever work, as all other queries will die with a concurrency violation until the first query has completed ( i.e. when the message is written to the message box). The UPDLOCK, READPAST locking hints are a proven technique for creating a re-entrant queue in SQL – here is one of many articles written on the subject http://www.mssqltips.com/tip.asp?tip=1257
Secondly, why do you output the SQL result to a file and then read it back in again. Why not bypass this step and have the subscriber of the file adapter, subscribe to the WCF SQL adapter.