You could get away with this, although I'd always favour a dedicated connection back off the 12v distribution. If you intend to connect the 12v permanent feed for memory retention too, this would really be on the only option.
In my experience I have had to replicate the ignition switch function. Without it even with the radio itself switched off it drained the battery. And as mentioned they have to have a maintained supply that is separate from the supplies feed via the habitation relay used in modern vans.
Interesting link there but the one thing I would be unsure about is how to route the power from under a seat to the wall unit and still hide the new wiring.
I didnt bother with the memory bit, I have to retune on most sites anyway, I still used a dedicated feed though, I got my cable though the wardrobe (with the rest of the van wiring)
The stuff that timtheenchanter has linked to is ideal and in fact some vans (mine included) use this anyway. It's neat and easy to use and you'll soon forget the stuff's trunked down from the cupboards to the back of the seats where you can easily hide and route cabling.
Screwfix sell this stuff in packs, but you might find they sell individual lengths in-store (they do with other pipework etc). The 'sheds' tend only to sell it in multipacks though.