Wouldn't that be nice? That's a great idea, but sadly, as of now you still have to jump through each company's hoops.
I find it handy to keep a folder on my desk top with various writing samples that I can just cut/paste into the field when I sign up for a new shop.
As to various certification quizzes, I'm lucky that I enjoy doing them. I like games and puzzles and I view them as just another brain teaser. If they give you trouble, try keeping two browsers open: one with the quiz and one with the "training". If it allows, read the quiz first so you know what to look for while you're reading the training materials. Since I find the pace of most videos way too slow for me, if the training includes text of audio, I shut my sound off read the text, fast forward and keep reading. I can get through it much faster if I can just read it myself and comprehend more. The slow pace of the videos causes my mind to wander.
There are two types of people in this world: Those who can extrapolate from incomplete data.