new body?

Have you considered doing a universal cut on the body to basically reduce weight and maybe make it more acoustic?  90's american strats have them and I believe it takes away from sustain but increases volume if that makes sense.  Maybe even take it one step further and drill weight relief pockets under the pickguard.  My American Stratocaster has a problem with that too, but doesn't have a big steel block or the universal cut.  Plugged in its what I demand out of any guitar all bussiness and nice and quite... GFS 60's overwound pickups.  Warren Haynes said the same thing when he is shopping for a guitar that electronics are replaceable but if your guitar doesn't sound good unplugged you'll never be happy with it.  I believe it because my Baja Telecaster I just bought came in and the first thing I did with it was play it for an hour unplugged to decide if it sounded good.  It had a return policy on it so you better believe I would have shipped it back to where it came from if it didn't ring like I wanted.  Oh one other thing, do you have the  bridge floating or hard mounted to the body?  Strats are bad for loosing tone due to the floating bridge, and before you go drilling on your guitar or swapping bodies I would try that first.  Either tighten the springs all the way, or get a 1 inch block of wood and wedge in behind the tremelo block like clapton does.  It makes a huge difference in string transmission to the body and sustains a lot more like a hard tail.

yeah the swimming pool trick.  It might be worth a try, sounds like your already doing everything else I thought of.