Rather than making a custom back as described at Wishlist:Expansion_Back, spacers could be used instead. They would be snapped onto the phone in place of the back, and then the original back snapped onto the last spacer. The user could add as many spacers as desired.
All spacers would be identical (although perhaps available in two colors, to match the current Neo color choices). Only one part needs to be designed and manufactured, and users can make their phones as fat as they need to for their expansion/hobbyist projects.
It may be possible to design one spacer that will fit several generations of Neo hardware.
Very crude artist's conception of a spacer:
One spacer being inserted between the phone and its back:
(The battery has been removed here)
One spacer already snapped into place:
Two spacers snapped into place:
For aesthetic reasons, the thickness added by each spacer could be made identical to the width of the phone's long narrow "side panels" that hold the USB connector, the earpiece jack, the two side buttons, and the two speaker grills. (I think this is several millimeters -- anybody got a more precise measurement?)
The spacer could be made from the same material as the body of the phone, so the color would match. Only the edge of the spacer is visible after the phone is reassembled.
Manufacturing such a spacer would require a simple two-piece injection mold (perhaps? would there be sufficient overhang in the design to require a more complicated mold?). If the expense of injection molding cannot be justified, they could be carved from a sheet of material (almost any radio-transparent carveable material, including wood) with a small CNC machine.
Small breakaway tabs with holes in them (not shown here) could be situated along the inner walls of the spacer to allow components to be fastened securely.
The user could drill holes through the side of the spacer to allow for connectors and buttons to be installed.
Shown in these images are two holes in line with the existing Torx screws. The original Torx screws could be removed and replaced by longer Torx screws to hold the spacer(s) securely in place.
Every Spacer should have ready to solder pins connected to a chained bus system like the I2C or/and perhaps USB and power connectors inside. So someone can easy solder and glue his extension into a spacer and the spacer connects it properly when attached to the phone. The back can become some functionality too, just by connecting to the bus system. See Wishlist:Expansion_Protocols for more details.
Ideas: An RFID reader/writer with I2C bus should work away just soldered to the pins of the spacer and controlled by a little driver.