Back in 1980, I not only cracked/broke the wrist of my NA Brown Bess Carbine, but shattered pieces of the stock off from the wrist. The minute I heard the stock break, I was heartsick. (I had made the mistake of inadvertently using the buttstock to "break my fall" when I went down into a prone position as I had been trained in the modern military.) Since I had begun doing WBTS reenacting, that Bess stayed in that condition until "I could get around to fixing it." What I did not realize at the time, that Bess would stay that way for almost 20 years before I came back to reenacting the 18th century.
As others suggested, I used two pieces of threaded brass rod and drilled holes in the stock ahead of and behind the crack/busted out parts so that the rods would not show through after the repair was done. I had also learned to colour Brownell's Accraglas to come very close to the colour of the wood. I also found that to get the broken out pieces of wood to fit, I had to actually finish breaking the butt section off the stock. (That was the scariest part of the repair.)
When I "reglued' the pieces of the stock and wrist back together, I was fortunate that rear of the triggerguard and thumbpiece with screw pretty much aligned everything back together. (I did mold release the metal parts so I could get them out of the stock later.) I applied further pressure to hold the broken out pieces of the wrist by wrapping the wrist with surgical tubing. Then I waited with much consternation while the repair cured.
To my delight, I found there were only the tiniest traces of Accraglas that showed through, after I cleaned the stock up. I used a tiny point black permanent magic marker to continue the grain of the wood through the traces of Accraglass that still showed. Then I stained the stock lightly. At that point, I thought about making a sheet copper wrap to go around the wrist, but decided to wait and see what it looked like after I refinished the stock. Much to my relief, I found it was VERY difficult for even me to spot where the repairs had been made and I was the one who did it. (I showed that gun to many others and none of them spotted the repair until after I mentioned it and I had to actually point out where it was done to most people.)
At that point, I decided to put off making a sheet brass wrap to go around the repaired portion of the wrist and stock. If I could not easily see it and most others could not see it, I figured I could live with it that way. However, if the repair had been more noticeable or had it really bugged me, I would have done the "authentic repair" of wrapping sheet brass around it and tacking it in place "just for show."
Gus