Hi, we're working with World Possible to support RACHEL-Pi in our products (WDLabs). We have experimental software based on the NOOBS OS installer that will install the RACHEL-Pi WiFi server onto a USB drive attached to R-Pi. It's set up as a dual-boot solution: (1) boot Raspbian to provide easy setup of WiFi and enable simple selection and download of RACHEL modules (just drag 'n' drop rsync commands from the RACHEL module repository, dev.worldpossible.org/cgi/rachelmods.pl, to the terminal) and (2) boot RACHEL-Pi to serve the downloaded RACHEL modules to local clients via direct WiFi. The modules are stored in the NOOBS "Data Partition" which is shared between Raspbian and RACHEL-Pi and is maximized to the available space on the drive. The software is delivered via 2.3GB zip file for the SDcard. The USB drive doesn't require preparation. After installation the SDcard is only used for the initial boot process (and for software reinstallation) and the USB drive handles the rest.
There's a PDF at the link below with instructions, link to zip file and link to a quickie video stepping through software installation and use of server/client. This should be considered experimental, though I've been working with it on various devices with no issues (e.g. Pi3, CM3, PiDrive 314/250/375, PiDrive 64GB Flash, etc.).
https://www.dropbox.com/s/pfly5lec9gm46 ... 4.pdf?dl=0
If anyone's interested, we'd like to do field testing and would provide beta test kits.
Dave Chew, WDLabs