The nice thing about prototyping is that you don't have to be backwards compatible. I've made a couple of updates to the design for the next (and hopefully final prototype):
1. I've added a 3.3 volt power supply to the shield adapter. LCDuino itself has no use for 3.3 volts, but it occurs to me that there may/are lots of shields out there that do. The 3.3 volt supply can be driven either by the internal 2.1mm VIN jack or over the I/O cable from LCDuino's USB connector. IOREF will, of course, still be +5v.
2. The LiquidTWI2 library doesn't actually come with support for a 6th button... Instead, it supports connecting and driving a piezoelectric buzzer from that pin. So I've swapped out the "back" button on the button board for a suitable piezo element. I've also added another row to the button connector (to make it 16 pins), adding a +5 volt pin, in case you want to design button boards that need to be powered.
Meanwhile, I've successfully installed a bootloader and an initial sketch on the prototype LCDuino. The first sketch is a GPS clock sketch that I've got running on my Uno. It's not actually connected up to any hardware, so the LCDuino just puts up an error message, but it does prove that control over the LCD, its backlight and the ability to upload sketches as if it were a real Uno.
I'm trying to work out what to put in the real "hello world" sketch for LCDuino. It's going to be something I can use also as a final check-out sketch to make sure everything on LCDuino is working, and something you can play with the minute you unbox yours.