This is a very intriguing idea. I wonder if it might be possible to connect the Seeed Studio XBee Shield (http://www.seeedstudio.com/depot/xbee%C2%AE-shield-p-419.html
), through the ISCP headers instead of the TFT expansion headers? If at all possible, this would be a major bonus as this shield, like the Esplora itself, is widely distributed (in the U.S at least) at Radio Shack retail outlets.
The shield has a small footprint and is meant to attach to a standard controller with a full ISCP Header and a single row header but fits nicely on the Esplora using only the ISCP- (See Attached Photo-https://picasaweb.google.com/lh/photo/sB5WVVc86AXqcR_F13Qwz9MTjNZETYmyPJy0liipFm0?feat=directlink
The only major downside is the same problem that affects any product produced by Seeed Studio... The extreme lack of documentation! The product wiki page- http://www.seeedstudio.com/wiki/index.php?title=XBee%C2%AE_Shield
still officially applies to version 1.1 even though the current version is 1.2, and while differences between them appear to be minimal, the documentation is confusing in any case...
I've never used the ISCP headers, or SPI Communications at all for that matter before, and from what I've been able to gather, the Leonardo has an altogether different pinout of these headers anyway.
So is this a possibility? It would make your suggestion significantly easier to deploy for a great many people!