I'd also put some feelers out on the trade front...maybe someone feels like you're selling low and they're buying low, so they'll give you something good for Byrnes. Even if it's a 10th-12th round draft pick, that's still better than what you could pick up off of the waiver wire.
Fantasy baseball's all about manipulating perceived value, so if you can get some guy who still sees Brynes as a top-100 player and is willing to part with a top-120-140 player, but you think he's basically not even in the top-250 anymore, then do it.
I personally don't have much hope for Byrnes. His career stats aren't terribly promising, he's usually a 265-270 hitter with mediocre pop (20 HR or so) and decent but not crazy speed in the 20 SB range. He's basically Mike Cameron with another 20 points of average, and personally I think both are going to wind up less valuable than Bruce in the remaining months of the season. But don't drop him until you are absolutely sure that no one will trade you a player better than what's in the free agent pool.