Wearing a weightlifting belt at the computer
 

I can't remember where exactly I heard of this mentioned in passing, or even if I remember it correctly.

But does anyone have knowledge or thoughts about using a weightlifting belt to support posture when forced to sit for extended periods of time e.g. if you're working a computer/desk job? Could it be useful?

Have you looked into Esther Gokhale's work on posture, and sitting etc?
Totally changed the way I sit, helps relieve alot of discomfort if I'm seated for long periods of time: www.gokhalemethod.com
To answer your question however, I'd imagine using a weightlifting belt would be seen the same way as using orthodics in your shoes: pointless.
It's too much support, so the muscles necessary to hold up your back (or ankle in the case of orthodics) become weak and will never get used to holding themselves up..
That's how I see it anyway..