From a little researching it isn't clear whether it is just tkinter that doesn't support alpha, or if TK itself has issues...but I'm hoping it is just the former (I found discussions of alpha for images, and it sounds like the Tk canvas supports displaying such images, but I couldn't find anything definitive about non-image support for alpha values).  If Tk doesn't have full support, you'd have to take a step farther back and get the needed RGBA support into TK itself.  Using a graphics package other than TK is a non-starter for the standard library.

So, assuming TK supports it, this is two feature requests: tkinter alpha channel support, and Turtle RGBA support.

As a feature request it can only go into 3.5.  The tkinter piece should be split out into another issue, with this one depending on it.

There are some people interested in improving tkinter (mostly in the IDLE context currently), but unless you can submit patches yourself probably the best hope for this is for it to get picked up in the GSOC or OPW context as a student project.