Regarding broadheads for larger diameter shafts, I haven't found much, either in historical replicas or modern big game points. You can get medieval style broadheads at a few british sites like http://www.longbowandarrow.co.uk/bodkins-18-c.asp , they don't list the weight but they look pretty beefy. They're expensive, and are either socketed for more normal size shafts or have a spine-needle mount which i don't care for. If you search the web for "Big Game Broadheads" you'll find a few vendors selling some awesome looking broadheads up to 200 gr, but again not "chambered" for 1/2 inch. Of course you can always barrel your shafts.
Not long ago I dug a couple of unique points out of the bottom of my archery tool box, where they had languished since I was a boy. They both had unusually wide sockets, and were big and fairly heavy. I mounted them both up to create one-off "Hollywoods" - I'm no hunter, these are bolts that just look really cool on the track when photographing my crossbows.
Your question got me thinking, so I did some research. It turns out they are 1950's era vintage: 3 inches long, heavy steel main blades with with thick plastic sockets that have a terminial outside diameter of 7/16", just about perfect for a 1/2" shaft. One is a Hilbre, with additional small "bleeder" blades, and slightly concave edges on the main blade, while the other has no brand marking, no bleeder, and slightly convex blade shape. I didn't bother weighing either one before mounting them, but they still felt a little light for the 12" oak dowels I use for half inch shafts, so I added a section of half inch aluminum tubing just behind the broadhead, which toughened up the business end, moved the center of gravity forward, and, most importanly for my initial purposes, looks cool. In retrospect, a shorter section of steel tubing would have been more effective.
This is a shot of one of those bolts, the one on the bottom, used as a size comparison for some pistol-sized broadheads I whipped up.
The total weight on this one is 55 grams, or 848 grains. Hope you find what you're looking for.