I find Gareth is very good at including notes on colloquial/dialectal forms in his grammar books anyway (compared to other grammar books I’ve come across which now gather dust on my shelf!), so I’m sure this won’t be an exception - in fact I’ve got a feeling that it could well contain more than usual going by the hints here and there in his posts in general across the forum 
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There is a sneak peek at the contents list/index here:
More SSiW mutational mirth after this short commercial break
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