I've had three different Buffalo NAS's over the years, and have occasionally followed the user groups that hack them. You're right that they run a paired-down Linux OS. And people in the nas-central user group have managed to install other, more complete, distributions. But it's always one of those things that seems to be fraught with peril. After retiring my old 100baseT 160GB unit years ago, I attempted to follow the online instructions for jail-breaking it and installing a more open Linux dist I ended up bricking the thing. It was no great loss; I just pulled out the drive and gave it some other use. Later, my retired Gigabit, 320GB unit was successfully jail-broken, but the results were never all that useful. I don't intend to mess around with the 2TB unit I'm using now. I certainly wouldn't suggest diving too deep into the unknown on a unit that I'm still using for anything serious. Data loss is a high probability if something goes wrong.

My suggestion is to get your hands on a Raspberry Pi if you want to tinker like this. Also, the Synology NAS's seem to be designed more for hackers.

BTW: I too noticed that current firmwares for the Buffalo products have Perl available, but if I recall, it's a 5.8.x version, and the last thing I would do on a NAS that has important data on it is mess with upgrading the system Perl.