You could sew some durable netting into the armpits; but only for a windshirt. I have a Montane windshirt that does this underneath the upper chest area rather than under the armpits, but it also has a loose, breathable DWR fabric cover over the netting.. When making openings in tent walls or garments, netting is sewn in to retain the shape of the item, especially for tent vents. Sierra Designs netting is used because it is much more durable than super light insect netting, and the weight penalty from a small gusset of netting is minor..
But for a rain jacket, as Mike B said, you need a zip closure over the pitzips for downpours if you want to stay dry, especially in blowing storms. The netting could also be retained under the zip closure to keep out flying insects, and possibly crawling ticks, although the ticks seem to be able to crawl anywhere once they get under clothing.
But I think enough needle size holes to provide good venting would weaken the garment. Even a windshirt can receive a lot of wear and tear, and moreso for a rain jacket.
On the other hand, if you can afford to replace gear often, stab away with a hot needle to limit unraveling. But Roger Caffin has suggested a better solution with a poncho that is short and tailored to fit well enough to resist heavy flapping. He has posted threads about it, and many posters have agreed that it is the best solution for those who perspire considerably. There have also been comments about Paramo garments: but since I’ve got a low metabolism, Pagatonia WPB garments, like the light M10, work fine for me in storms. I did have a “Ventile” tightly woven garment, but it was no better at keeping me dry, both from within and without; worse actually than something like Roger’s poncho.
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