Pronunciation of '-ing' endings as '-een'

Recently my daughter told us she’d had a supply teacher at school because her regular teacher was “at home pukin’.”

The pronunciation of the -ing ending as in’ [ɪn] (rather than [ɪŋ]) seems to be fairly region-dependent, and often appears in North America, if I’m not mistaken, in rural areas such as the one I live in (and in researching this question, I see that the pronunciation has long historical roots).

My daughter’s pronunciation got me thinking about another pronunciation of the -ing ending that I have noticed occasionally throughout my life: een [in]. (“I’m walkeen to school,” or “Why are you runneen so quickly?”). Unlike the in’ pronunciation, een does not appear, to my observations, to be dependent on either region or class, nor even generation. It doesn’t get caricaturized in song lyrics or on greeting cards. It doesn't even seem to run in families! When I’ve mentioned this pronunciation to other people, many haven’t even noticed it (“She says runneen? I never noticed before.”)

I don’t think I have heard a Brit pronounce it. But I have certainly heard Canadians and Americans from various ages, social classes and regions pronounce this een ending consistently. A few, not many.

Am I right in thinking that this pronunciation is not associated with any particular region, generation or social class?
If so, under what circumstances does it occur in a person’s speech?
Does it occur outside of North America?
Does it have any history?

While researching this question I came upon this Language Log post in which Sarah Palin’s pronunciation of “shackling” as ['ʃæklin] is much discussed (but without answers to my questions). There is also a recording her pronunciation.

EDIT

This Language Log post appears to address this phenomenon. The author concludes as follows:

[As far as I know] there are no systematic studies of this range of
phenomena. As I mentioned, it's part of the folklore of
sociolinguistics that the [in] pronunciation exists, and that it's
(apparently) not stigmatized in the way that the [ɪn] pronunciation
is, and that in fact it may be heard as [ɪŋ] or [iŋ] and even
transcribed that way in some studies. But given how much attention has
been (for good reasons) given to g-dropping, it's odd that this angle
has been so neglected.

I'm not sufficiently educated to understand the post that precedes this conclusion, but I thought I'd try to point other users to it in case they can make better heads or tails of it than I.