I just bought a NAS, specifically a Thecus N2200EVO, to do LAN music streaming and some other miscellaneous storage tasks at home. I was just on the verge of disappointment at a few minor niggles, when I discovered the SSH interface.
It lets you log in as root. It’s Linux on ARM. All the partitions are writable, and /etc appears to be mounted on a persistent storage device. The web interface is written in PHP. Within a few minutes, I had hacked out the annoying Flash login page, now it is replaced by an HTML fallback (at least until the next reboot).
The startup process is implemented in shell script, with explanatory comments by people called Astro, Michelle and Angus. It uses SQLite for configuration storage, in addition to plain files.
I understand from a quick bit of web surfing that it is trivial to write your own “module”, which can contain modifications to the PHP web interface, arbitrary cross-compiled binaries, etc. Modules can be installed via the admin interface and are stored on the attached hard drives.
It’s hard to imagine a consumer device offering an easier path to customisation for a LAMP developer like me.