I would like to share the method that I use to change default stems to any type of stem in a very simple way. The only condition is that you don’t need to use some of the default tremolo stems. The method consists in changing the default symbol for one-note tremolos or buzz roll (Engrave>Music Symbols>Combining tremolo # or Buzz roll) to any stem or symbol you like. You have to align the stem at the centre to avoid a doubled stem:
This method does not allow define a stem that can be flipped, though. I hope it will be helpful to somebody.
By the way, I would like to ask for this feature as default… Being able to apply the magnificent stems in bravura to any stem would be great!