(WARNING: DO NOT ATTEMPT ON v4 FIRMWARE)
Disclaimer: I take no responsibility for your device getting bricked as a result of proceeding with this, so take caution!
Decided to do an experiment today. After bumping across http://snapshot.debian.org/ followed by some extensive repo switching on /etc/apt/sources.list, I managed to find a working repo entry that will play nice in installing build-essential on myCloud v3 (last version) firmware.
deb http://snapshot.debian.org/archive/debian/20131130T095229Z/ sid main
Because release list is expired, you need to write the following to allow apt-get to process this repo:
apt-get -o Acquire::Check-Valid-Until=false update
Once complete, simply write
apt-get install build-essential # or alternatively (I used apt-get, but aptitude was useful for tracing dependencies) aptitude install build-essential
At this point, it should be safe to install, as long nfs-common-kernel or whatever (usually as a result of being made to upgrade/downgrade your version of libc6 which this WILL NOT do) is not on the list (which as far as I know is one of the causes of the bricking since it was manually installed by WD). The whole idea is to install the bleeding-edge (of the time) build-essential environment such that libc6 etc. have the same version as that installed on the myCloud firmware.
Sample log (note: your list may vary since I have already previously installed other applications):
EDIT: Update - Log output installing build-essential straight from a FACTORY RESET v3: https://dl.dropboxusercontent.com/u/1287967/build-essential-mycloud_FACTORY-v3-sid%202013-11-30%20T%2009-52-29.txt
Obviously what this means is now it is indeed possible to build from source directly from the myCloud, without having to set up a separate Linux environment to doing any cross-compilation! So far I haven’t seen any issues
Hope this helps anyone!
EDIT: Output installing clang right after, from the same repo.