I wouldn't feed woodchips, but small, green branches can be utilised by
cattle. In the book Man, Cattle, Veld, the author (sorry, I can't remember his name and the book's packed away, but it's Johann something) talks about cattle browsing
trees and that they are supplemented with polyethylene glycol to ameliorate the effects of the tree tannins.
Here's a link to a study using poly glycol in the Mediterranean
Effects of Polyethylene Glycol on Trees with Tannin
I've also read, though I can't remember where, that trees will increase their tannin levels when surrounding trees signal that they are being browsed. Some ranchers pulse their cattle through the trees, only leaving them in one spot for 24 hours, then moving them before the tannin response can happen. This way they avoid the high tannin levels and don't use the poly glycol.
Personally, I can't see ever using the poly glycol, but Walter Jeffries has said that his pigs will munch on young brush, and Travis' comment is spot on, so I wouldn't rule out using young brush as a supplement. I wouldn't try wood chips, though, unless it's chipped brush.
I can't recommend Man, Cattle, Veld
enough. It's got fantastic info on rotational grazing, grass quality and especially helpful info on genetics, breeding and hybrid vigor.