c163,
I am definitely a N00b as far as FreeNAS is concerned so can't say for sure if it's "possible" to do, however, I have been using MythTV for a number of years now (around 4 if my memory serves me correctly) so can hopefully help from that standpoint.
Apologies if your already well versed on MythTV......
MythTV is a big product. The dependency list is huge. I use Fedora Core as the base for my installation so I am lucky in that binaries are pre-built for my system and I can just run a "yum install MythTV" and away it goes (after altering the yum repository settings). I have no idea how FreeNAS would handle this. You would have to download all of the binaries and install manually I would imagine. Then there is the question of what you will use Myth for. Recording TV ? If so, what source ? Free2air (Freeview), Freesat, Satellite ? If you are going to record TV you will need to install a PVR/TV card. If you want to record multiple channels, multiple TV cards. You can also use Myth as the center of a CCTV system (Zoneminder), will you want to use that ?
The following links will give you an idea of the per-requisites and requirements for a Myth install
http://www.mythtv.org/wiki/Hardware_Requirements
http://www.mythtv.org/docs/mythtv-HOWTO-3.html
What I can say for definite is that you can use a FreeNAS box for data storage for your TV/Music and Movies. Which is how I use it. I used a USB 1TB hard drive for a while until it failed on me. FreeNAS is a much safer proposition.
Leenux_tux