I love the idea, but I think it's a bit ambitious right now. Work should probably be focused on developing the app and site more first in my opinion.
Viewing post in ItchOS
Well, it could be simple as build up a regular linux install (set as regular user things such as: default applications, ppa, driver, wallpaper/themes etc); then backup the whole disk (all the partitions inside) into a single downlodable package.
Then, user may only need to do something like
gunzip -c ~/Download/ItchOS.img.gz | dd of=/dev/sd[usbkey letter]
to "install" the system onto a usb.
But there's no doubt that website and app should have aboslute priority.