Okay, for anyone who has been following this, I FINALLY got it completely solved. The sixth field is indeed used in computing the block hash. It turns out this is actually a well know addition to stratum mining - but I didn't know about it and no one I asked mentioned anything about it. I looked more closely at the traffic and noted an exchange where the miner and pool negotiated something called "version rolling" where the pool responds with a "version rolling mask" which indicates how you can manipulate the version number before using in the block header to hash the block header. The advantage is that rather than mess with the extranonce2 value (which you are still allowed to to if all else fails) you can mess with the version instead. This has the advantage of not requiring recomputing the coinbase and merkleroot so it saves a bit of effort. The version rolling mask tells you which bits of the version you are allowed to mess with and which you are not. That sixth field is the version number you used in place of the orignal version number the pool issued with the job.

So the solution after about a month of investigation was to do exactly what I was already doing, but to replace the version number of the block with that sixth field (well, not quite - you need to combine the mask, original version and sixth field together in a Boolean equation to get it right).

Anyway, after a month of spinning my wheels, I finally generated a lovely hash for my accepted share with a long string of leading zeros on the front of it.

By the time I sorted things out, I became very proficient in building Merkle trees and branches and messing with the parts to make the coinbase, and even found out how something called the "midstate" can be precomputed for 64 of the 80 bytes in the header so that replacing the nonce in the reaming portion and re-hasing becomes far more efficient as nonces are tested. So I guess it was a blessing in disguise that this was not strightforward to solve. Although, in the end, it turns out to be pretty easy - once you know what to do!
-gt-