I've caught back up on this thread, and a few thoughts came to me:
1) I think a real consideration vis-a-vis some sort of 'Subway bracket' is how many 'sheesh, I just pulled that 5# head off the cabinet and now it's toast' warranty claims (or 'your fault' or out of warranty repairs) have cropped up. After a certain number or steady percentage of these claims or repairs is exceeded, it would begin to make a LOT of sense as a preventative measure.
2) I still have a soft spot for combos, and have been looking at the little Tech 21 200w combo and its matching extension cabinet. This is may favorite layout and compromise of size and power.
BUT . . . . if I'm Mesa, and we can sell old JW his preferred Subway head and two single 12 bins (the same similar single 12 bins with an extra box on top, just like the Tech 21 rig) with the resulting gains in power, tone, etc., why would we worry with building something like that when it will probably be heavier, probably less versatile, and probably more expensive? And will it cannibalize Subway separates sales and/or will we sell enough of them to make it worth our while? Oh, and PS, we got to make this fly against Rumbles and the new Ampeg combos. This hill is getting steeper to climb by the minute. . . . .