This is an awesome project, milesj: great the idea of decoupling scripts, CSS and HTML, the minimal styling (it reminds me of the approach behind the underscores base theme for WordPress), the concerns for accessibility, even the project of adding a LESS version for a future release! Great job and well-written article.
One small concern for starting to use your framework would be: after taking care of it for so long, is there any assurance that you’ll keep supporting it for at least the same number of years? I know you mention a future release, and I know this is a catch-22 situation: the more people adopt Toolkit the more chances there are that it’ll be fully supported, and the more the chances that it gets supported in the future the more people are encouraged to adopt it.
I wish your framework a great success and will start seriously looking into it for my projects 