I have a Widescreen TV but haven't actually tried this, however all widescreen TVs do is stretch out the horizontal direction. You would need to force it into 16:9 mode as the Amiga doesn't output a widescreen switching signal, but this shouldn't be a problem and I imagine it would look like a wider hires mode. I've been meaning to try it for ages but my Amiga is happily connected to an ordinary monitor and it will probably stop working if I disturb it.
Chris