Any question answered...

Because I dont have 1, have some of the worst lbs choices in history, and not sure how to measure the grade of bolt that would fit.
And there's plenty of space because of an extended seat tube

m8 you say, interesting...

How would I measure the grade apart from trial and error?

I guess it is more likely to be M6 and you are able to get a thinner M5 bolt in the hole, which is your temporary solution. A hardware store should have a suitable AK headed steel bolt, and it doesn't have to be a high tensile bolt, it isn't under that much tension. Just use a standard steel bolt and if it hasn't got any corrosion protection (i.e. it isn't stainless or plated) make sure it has plenty of grease on the thread to protect it from rusting.
The bolts that hold cantilevers or v brakes to the frame or forks are usually the same thread, any bike shop, no matter how shit will have one somewhere.
Even with a lot of seat tube you will still have to get the QR clamp over the brazed on clamp, which could be difficult.