lol! funny idea!!!it s unfortunate I can t know first hand, as there are no lego store anywhere nearer than...yours I guess, [I live in quebec], at least, I have a lego EDUCATION dealer in montreal, and with their line of products, I consider myself quite lucky! do you know about them paul?
The LEGO Store pick-a-brick cups are made by lego and made to stack just like Lego bricks hence that makes them a LEGO Product! I think it ingenious to use them this way!
Paul use one more row - instead of 2 use 3. Use Epoxy glue to the caps to the cup and then on each interlocking cup.cap set that will give it stability.
Throw a few LEGO Basic bricks in each cup before you glue the caps together. Then drill a hole and add wooden rods (dowels) to interlock each row!
Add a cushion for a seat and "by George I think you'll have it!"