OK, so it is possible, nay relatively straightforward. If only connecting through BTOpenZone were so easy, or I would have reported back yesterday!!
It breaks down like this:
Arrive, dock at jetbridge in new terminal building, run to passport control (4 mins) -- there are two visa purchase windows, only the one on the left was open. Slap down 10 quid, get sticker, get in queue for passport control.
Wait (5 mins)
Proceed through baggage claim to arrivals, go to the right when through the door, head to the end of the terminal and go upstairs. At this point you will be directed outside the building, as all departing passengers get screened when they enter the building. Go through, bearing in mind that the Turkish magnetic machines make the US ones look inefficient and weak. I think the studs in my jeans set the machine off

(5 mins)
BA check in is at G11, right next to Swiss. I actually went there first, and they directed me to a window at the end of the line. The gentleman behind the window heard my name, reached into the drawer and flipped me an envelope with the magic 16 coupons in it. Not even asked for ID!! Went back to check in where the agent had already printed my BP while I picked up ticket. (3 mins)
Back through passport control, return to the gate where I left (5 mins) and boarding hadn't even begun. Looked around in vain for the lounge (apparently it's down some stairs just past passport control, but I'm damned if I could find it -- get GOOD directions, and PAY ATTENTION, unlike me

)
The whole roundtrip took a little over 25 mins (I messed around in the middle a little) so I sat back in 2C and came home again. Very enjoyable -- the new terminal is very cool, and so much better than the scrum which I went through last time I was out there.
So for those short on time, it's not that big a deal. I'm sure it gets busier than when I was there, but even so there's a lot of margin for error. The fact that my flight was 10 mins early ^ ^ helped a lot.
Cheers all,
Chris
PS spottie -- sorry I missed you by a day -- I'm flying up to GLA so they won't let me in to see you on your way out. Have fun (I hope you actually get to spend some time there...) CT