I'd love to be able to help, but my main concern is that it'll be adding extra, unneeded functions to an already perfect design, especially if those functions aren't anything you need in your normal usage.
Plus, I realised I haven't thought things out very well yet. While it might be possible to add a mini-itx pc under the tray - there isn't anything one can do without a power source, or for that matter, a screen. Not exactly something that you can use on the go, which eliminates a lot of the functionality, unless you do indeed intend to plug into a power socket and have a random monitor you can plug into at the places where you use your ergo-to-go. This applies for the raspberry pi idea, which also requires a power source - something that detracts from having a mobile solution. Upon reflection, this makes me feel rather silly for suggesting it for the ergo-to-go.
It reminded me of how an old Chinese saying goes - "adding legs to a drawing of a snake" - do that, and it's no longer a snake anymore, but a lizard instead. As it is, I think you're already on the right path, and my suggestions are distractions you should ignore.
I guess the only thing that I would actually want to do with a something like this, would mostly reside in the case design - in particular, a way to mount and hold a tablet. Like CPTBadass suggested, a usb hub is a good idea, especially for connecting the trackball and keyboard, so that you only need to plug in one cable from the hub to your pc and be good to go.
It's
a little very embarrassing to show my doodling, but I couldn't resist making a rough drawing of what I thought it should look like, internals, and perhaps a groove to seat the screen in (the one I have in mind is actually a surface pro, but I realise that you're a mac user, so options would be more limited). Its always been something I wanted to do in the past when the surface pro was first announced, even with a normal TKL board - to use a tablet in conjunction with a normal mechanical keyboard (but of course, I never really considered the mouse aspect then; I thought the touch screen capability would suffice.) With the ergo-to-go, it seems like a more complete solution - although the surface pro's battery life have been rather disappointing, a large enough deterrent for me not to get it.
Excuse the rough drawing, I haven't drawn technical stuff for years.
Basically, a laptop replacement using a tablet. A usb hub hidden inside the tray to connect to the tablet, serving as a intermediate connection to the trackball and the keyboard. There would probably be weight balancing issues with the tablet, and I'm sure you have much better ways to do it if you did intend to go this way.
edit: I like that lunchbox idea!
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