Sorry to hear about your break Timberghost, that's no fun, but it is definitely repairable.
With the grain as wavy as it is, it shouldn't be difficult to realign the two pieces perfectly. An oval wrist inlay in the grip would suit perfectly for hiding a drill hole and a clamping ledge. I would chisel out a small ledge in top part of the grip, then drill down through and install the dowel as suggested. Use an undersized dowel in the hole, one that would otherwise rattle around - this gives you some leeway for fitting the two pieces back together. Do NOT use cheap hardware store epoxy that comes in the equal-part syringes. These are fine for household jobs, but are not as high quality or as hard as marine-grade epoxy (order a small batch of West System online). Titebond 3 wood glue is a good alternative to epoxy. Cut your dowel to be slightly below the surface of your ledge you carved out. Fill your drilled hole with some glue as well, then insert the undersized dowel, and now you can use that same ledge as a spot to place your clamp. If you align things right, the clamp should sit between the buttcap and the ledge and pull everything in line without it sliding. After it's assembled and fixed, glue in a small chunk of wood to fill the ledge and install a silver or brass oval inlay over it - if all goes according to plan you'll never know there was a hole underneath it and your crack will be invisible.
Alternatively, If through your ledge doesn't seem feasible, you can chisel out a deep 1/4" wide channel underneath the triggerguard and place a piece of straight grain maple inside that, to be covered by the brass. I would still cut a ledge for your clamp to sit on and hide it with an inlay.
Good luck - in the future, as BJH suggested, avoid wood where grain runs out the grip - it's worth spending the extra money on a blank that has grain that curves with the shape of the pistol. I'm restoring a 1770s pistol right now with the same exact break... unfortunately the bottom half is missing.
-Eric
