the attached file is a first attempt to make feathered beams more variable.
p.e.:
http://old.nabble.com/file/p32705102/beam15.png
This is really cool! What a great function!!
Some problems persist:
The function is limited up to 64th notes. (I'm sure it could be defined
recursive to generalize it, but I didn't manage.)
I haven't tried to break your function :) but the attached file shows one way you could generalize it to remove the code duplication and work with more than four beams. (In the example, I've changed the first group to use 128th notes.)
The examples that Gould shows (pg. 158) have the peak of the feathered beams aligned with a stem. I think this would be a useful variation of the function. It shouldn't be hard to automate: (ly:grob-object grob 'stems) will get you an array of the stem grobs associated with the beam, and you could select a particular stem from the array with ly:grob-array-ref.
--David
harm-feathered-beam01.ly