I have a suspension fork, which has a stripped lower post mount, i.e. the bolt used to attach the brake caliper doesn't grip the post. Before anyone asks - yes, it is the correct size bolt, as the upper post is fine. Would it be possible to use something like a heli-coil insert to repair the thread or would you consider this unsafe ? FYI I am running a 160mm disc up front, and only do XC/Trail riding - so nothing too mental :-)..Thanks for any advice.
You might consider tapping this thread back out if you have a decent tap-and-die set. Otherwise, a heli-coil should work for this application. I would be more concerned with a heli-coil if you were using it on a post-bolt that would experience tensile stress--that is, "pull-out" force. This bolt will be under mostly shear-stress, which means that the majority of the strength will be derived from the body of the post rather than from the threads.
I'm not an engineer--engineers, please correct me--but this is what my thinking is when I fix a broken piece of metal in a situation like yours.
If you are a little paranoid, you might goop the bolt threads with some JB Weld before threading them into the heli-coil. This will add a little tensile strength and piece of mind, but should be pretty easy to break out with a breaker-bar if and when you choose to remove the part.
Note also that, in my experience, any use of a heli-coil or JB weld has the potential to be an "ultimate" fix. That is, once it's fixed... it's fixed for all practical purposes and subject to failure if "fixed" again in the same place. I might do this to my 90's Rockhopper, but I don't suggest it for your 2011 Santa Cruz Blur that you plan to change everything out on next season.
Short answer: With the sort of riding you are suggesting, I would do the fix you are suggesting on my beater mountain bike. On my high-end down-hill ride... I would take it to a frame builder and have the post replaced.