Fi I so know what you mean about wanting there to be a quantative scale for skin hyperextensibility, but it's not something I've ever seen (there may well be one more clearly in paid journals somewhere) However, as Nonya points out, I think this is one area that only really the experts can answer and that's because they're the ones who spend all their time doing this...I suspect it's one of those areas that's just too variable even within types to quantify in writing or even in pictures as it might then lead to incorrect diagnosis that way...but that's just my guess.
Antimatter, I had a look at the document on the Canadian site you linked to but as far as I understand both the diagnostic process and the wonky format/badly worded way they've put it there, I suspect you've overestimated the amount of skin hyperextensibility they are looking for. I know in the UK skin extensibility is often diagnosed in smaller stages such as 1+ or 1- the highest number I've ever seen being a 3...my suspicion again here being that if the higher numbers of the scale are used that a 4 or 5 would only be for those with types like DERMATOSPRAXIS where the skin is noted to be particularly sagging and redundant, again this is just my guess work.
However, having said that, looking at the scales you've written out, I suspect that actually the scale may be something like a skin stretch of under 4cm on the forearm in an adult is measured from 0-4 in increments of 0.5cm and anything over 4cm is recorded as a 5. Again I'm guessing but actually what seem to be very dramatic skin stretches in photographs probably don't measure anything near as much as they would seem when recorded by a doctor from the correct points etc. I know that in my own case, I have a diagnosis of EDS either written as global or just no type as I fit too many of the types. I fit mainly into the classical and then hypermobile types then with major features from too many other types to be given classical/hypermobile as a diagnosis, but have apparently mainly classical skin...which I wouldn't have realised as I don't think my skin is particularly stretchy...only diagnosed as a 1+ stretching maybe 2-3cm on my hand, around the same on my forearm, but can be stretched bucketloads (literally!) on my stomach. However what seems more important for diagnosis is that my skin is both velvety and papery and has the really strange doughy feel to it, which along with a few other issues put me into a classical category for skin features.
All that's a bit of a ramble, but it's me attempting to explain that I don't think there is a specific skin scale as such as I don't think they use one like that, at least not in the UK. They prefer to rely on a careful history and clinical exam to make their diagnosis as experience has proven that to be more accurate than any other method. It's also apparently why apart from a couple of centres in the UK no-one routinely biopsy's EDS patients, even the suspected vascular people I believe. It's apparently believed that with the length of time taken to grow the cultures, send them to Belgium, cost, not infrequent loss or corruption of samples etc that even when they do get the results they don't have any value over proper history and clinical exam. Apparently bloods/biopsy are only usually taken when there is a large family with known history to take from so there are samples to research etc, otherwise it tends to be once every 5 years or so that a blood sample or biopsy is taken at one of the UK's specialist centres. I'm guessing but guessing that the skin scales are similar and something that until such time there is enough funding available to research and publish properly then this is another area that is only outlined in the literature.
Having said all that (essay writing again!) I don't think it's that difficult a judgement/diagnosis for doctors to make once they've seen an example and seen these scales written down etc as of course they understand it fully unlike us lot! I saw my GP this week who told me he's now diagnosed his 3rd HMS or EDS patient since I joined the practice 2.5 years ago He said he feels it's a relatively easy condition to pick up on in general practice, and not difficult to get confirmed by rheumatology (he has someone decent to refer to) but said that it's just annoying no-one suggests the diagnosis to them as as soon as he knew and saw just one patient to learn from (me) he's started picking it up correctly, but in fairness to the medics, I know my GP has found it far easier to learn what potentially these issues may appear like from my body than he has from the photographs etc he's seen in the literature so perhaps it's a good thing there isn't a quantifiable scale as such laid out for them to follow as I suspect most of us would not be diagnosed from something like that as we would be considered 'too normal' in comparison.
Er, I hope my nonsensical thoughts on the matter have helped someone and if not...well I just hope they make sense!
"And the day came when the risk to remain tight in a bud was more painful than the risk it took to blossom" Anais Nin
Any advice is from my experiences with EDS. I am not medically trained