"Y'all'd've" . Great word, Mike. I love it. We should definitely make this italki's 'Word of the Day' for today.

And thank you so much for making two important points:

1. There's a difference between the contractions that we say and hear in spoken English - like 'd'you' and there're - and the standard ones - like 'it's' - that are part of the written language.

2. Anyone who peppers their writing with  'wanna' 'gonna' and 'ur' may as well be wearing a T shirt saying 'I'm a non-native learner of English who's trying to be cool but failing miserably.' 

Nobody listens to an old British schoolma'am saying this, but when it comes from young American guy like you, people might take notice. Good on ya, as the Aussies say.

There are a lot of such "contractions" imitating fast speech, dialects, articulation, etc.  in the books about Harry Potter. 

Examples:

'S' up, Figgy!

-Where' Tonks?  -Watcher, Harry!

What didja call Neville, Minister?

Ow come you di'n't tell us 'oo you are, eh?

"Righto," said Stan. "Old tight, then..."

We 'ad 'im 'ere, di'n't we?

Ouch - gerroff - gerroff, you mad old bat!

Some'n say m' name?

-Harry's arived.  -Blime, so 'e 'as.

Hasn' - hasn' anyone bin able ter open their books?

Now, firs' thing yer gotta know abou' hippogriffs is, they're proud.

... and so on.

Actually, it's a matter of trascribing words as we hear them.  But they are funny for giving us an impression of the livid speech.

Mike, Su.Ki.
a sidenote (and likely it is a good idea to remind about this from time to time.):
"like this should be used in written English" .  You specify: 'in written English'. This is really important point.
The realtions between:
- orthography
- written norm (besides spelling rules)
- spoken norm

Are very language-specific. For some learners from some countries the very idea that something can be less acceptable in writing is exotic at best (by 'writing' you mean texts like forum posts, not books as I understand). 
I see that this concept (acceptability of something in writing) is somewhat important for English. There was a time when I was unavare of this. I'm grateful to those wise enough to mention this word ('writing') instead of assuming it, I was surprised when I read it for the first time. And it doesn't mean I understand the relations between speech and writing  in your language. No, I do not.  In the case of "y'all" it doesn't seem perplexing from my perspective though:)

So don't expect the learners to have an intuition about what should be written and what should be spoken...   

In fairness, just like every rule there are exceptions. The one word like that that I do use a lot in text conversations is "gotcha". It just looks really weird to write out "I got you" and part of the reason you say it is to give a quick one word response (generally speaking). And in fairness, even native speakers do use them on occasion, but most (educated) people only use them sparingly in my opinion. A lot of English learners don't seem to realize that if you use those types of words a lot you'll sound like a teenage girl haha.  

"ur" and "imma" in particular drive me crazy. 

As far as audio clips of the pronunciation, attached is a link with some pronunciations. 

http://es.forvo.com/word/y%27all%27d%27ve/#en

And here're some clips for "y'all's" (which would be said even more often)

http://es.forvo.com/word/y%27all%27s/#en

It's kind of funny though, because most of the people pronouncing those words have sort of a strong country accent. They're not helping my case haha. As a general rule though, I think that the faster that they're said, the less country they sound.