Hi, @iancox, I’ve just sent you a big pull request which adds several features, including RGBA (“opacity”) functionality. Other features include the ability to select the type of colour picker, and being able to add pre-defined clickable colour swatches.
I came across several interface update issues, and eventually overcame them by following some advice from @bastianallgeier and refactoring the entire javascript as a jQuery field plugin. The new code is not minified, and fully commented - so it should be easy to update when new versions of Minicolors come out.
I updated the readme to reflect the changes in functionality, and also thought it might be a good idea to upgrade the version in the package.json to 1.3, so people are aware that there are new features. It should be backwards compatible with blueprints and templates using 1.2.
Please feel free to test everything - it is not thoroughly tested, but I found it stable enough for me to use it in a couple of my production sites already - and my users like it. 
I hope you might find it useful!