My requirement is to fit under the seat in front of me in Seat A of an RJ, and still leave room for my feet. I'm tall enough that even exit row is cramped for me.

For those fortunate enough to be unfamiliar with Regional Jets, there is just one seat on that side of the aisle. The space under the seat is narrowed by the curve of the fuselage, and the opening is roughly square. Cases that normally would fit under the seat simply will not lay flat in that space. You need a smaller case, and I need one that sits upright, leaning against the curved side.

I use a small laptop bag that comes as a perk at a conference I attend every other year. I use these bags for everyday use, as well as travel. Each one lasts about 4 years.

Just got a new one and it has the scanner-friendly ability to zip open and lay flat.

The case easily fits under the seat upright, leaving room for my feet to fit under there too.

Handles slide neatly over the handle of my roll-aboard and it has loops on the back for folding umbrella (actually two sets of loops, so I could tuck gloves and a watch cap in, as well). Comfortably holds my 17-inch laptop, powersupply, mouse, security cable, digital camera, iPhone, and associated cables. Nice expandable pocket for business sundries such as cards and pens, as well as my car keys. Inch-thick zippered pocket for file folders and such. And one open pocket for more papers, such as itinerary, boarding pass, and the morning newspaper; can stuff a half-liter water bottle in there, too. And one more small zippered pocket that provides convenient access to necessities such as kleenex, toothbrush etc. Also use that pocket to facilitate a walletectomy, to make sitting more comfy.

Edited to add: sorry to say you cannot buy just one, would have to be bought in quantity: http://www.brandedgear.com/ They have about 70 variations on the theme.
The above-referenced Mobile Edge Scanfast looks as if it would meet my requirement, and obviously is higher quality than the trade-show perk, for which the conference paid about $10 per.