getPropertyValue for custom properties appears to be disabled in Firefox Aurora which is the only browser that does support the latest variable notation.
Interestingly Chrome doesn't seem to even recognise properties with doubled dashes at all with code like:
var root = document.querySelector(':root');
Both browsers behaviour appear to be correct as the spec suggests: http://www.w3.org/TR/css-variables-1/#apis
Note: Custom properties do not appear on a CSSStyleDeclaration object in camel-cased form, because their names may have both upper and lower case letters which indicate distinct custom properties. The sort of text transformation that automatic camel-casing performs is incompatible with this. They can still be accessed by their proper name via getProperty()/etc.
@tabatkins the quote above should also say
I saw your talk on flex and grid, I can see why you were most excited by CSS extensions now .
Thanks both for your help, please let me know if I am missing something.