Although I have never tried this; I do know of quite a few brewers who have caramelized beers. I believe this is also the procedure Timothy Taylor uses; but I have also read that they just add caramel.
Basically grab about 3L from your brew kettle and add it to a pot, then boil it quite hard to reduce it back to under 1L. This will caramelize a lot of the sugars, giving that toffee flavour. This just added back to your original kettle, which is most likely finished it's 60/90 min boil.
I guess you could even go through the normal process of making toffee from sugar, I wouldn't let it get to thick. Then just throw that into the kettle.
QldKev