Just tested it. Looks like shuffling by "comment" works as long as the numerical value is the first thing in the comment. However, that wouldn't do the 45% 5 star, 30% 4 star thing because shuffle just shuffles an existing playlist. It doesn't create new playlists.

So what you do is, you set up a custom shuffle that has a small random element but favours least-recently-played, highly-rated songs. If you get the coefficients right, you should just be able to get into the car each day, play all with shuffle turned on, and you'll get the effect of a different playlist each day that's mostly 5-star songs, a few 4-star, the rest turning up just at random, and so on.

Custom shuffle modes, in some sense, are auto playlist generators.

Peter