I like it Mo! I have in the past taken a V-Tool to carve a swale like divirter under such circumstance, and will add this bit of thinking to mortises which could see wind driven rain in the future.

Doing restoration with regularity, such causal roots of rot are always high in my mind. I see posts lost to pinhole leaks directed into brace mortises with some frequency, this incubation often exacerbated by shredded materials introduced by mice when setting up nests in that empty square part of a brace mortise above the tenon.

As an aside, when sucked into the whole slope vrs no-slope debate, I have pointed out this potential and how I have seen leaks in brace mortises without such sponge-like nests (often scribe era sloped ones without room for such - Yes they do exist) fare far better - And have suggested that I always put slopes in barns (every Barn hosts mice) for this reason.