Well, I have lots of /echo commands in my macros, and if you could somehow filter the "Equipment changed" message out, you would likely filter out the /echos, as well.
Additionally, I'm not sure how useful changing before each shot is. When you're using lowbie ammo, and all of your stacks are loose, that's great and fine. Once you hit 14, I would assume you'd be using quivers, which makes this macro pointless. Once you run out of loose arrows, you'll have to use some quivers anyway.
I personally set it up so that I have 2 loose stacks of arrows, and I do an ammo change before a WS. This way, I'll never miss the WS, and the only time I'll run out of arrows is when firing normal shots, in which case I just hit the following macro:
/item "[type] Quiver" <me>
/equip Ammo "[type] Arrow"
Even if you use the re-equip-every-shot macro, you'll need at least two loose stacks of arrows, and there's functionally no advantage over my method. In fact, using your method, it's possible that you would have to use quivers twice in one battle, because both of your stacks should run dry within a couple of shots of each other. I'll never have to use more than one quiver per battle, as I do the ammo change straight to the fresh 99 and stay there until the next WS.
Terraxia - RNG/COR/THF - Midgardsormr/Quetzalcoatl (boxed)
Viper Beam - ARC/CRP - Fabul