I've tapped and plugged the decomp hole on my puch kit, but you're right, yours is WAY closer. While in my head, I don't think tapping it out and plugging it would put weird stresses on it like the polini intake problem because the hole you're tapping would be parallel to the cylinder bore as opposed to perpendicular to it. That said, things that 'work in my head' are not something I'd expect you to try on a cylinder that costs over $100, and probably more.
Just another idea to try, would be to find a small piece of rolled metal bar stock that fits in the hole, and either epoxy it in, or like you said, put a lil' dab of weld at each end to keep it in there, and then grind, mill or sand it flat.
Wicked portmap, BTW. What kit?