Hi Paul,
I'm sure you know of the override pair:
\override Stem.stemlet-length = #0.5
\override Stem.details.beamed-lengths = #'(4.75)
( music with beamed rests here )
\revert Stem.stemlet-length
\revert Stem.details.beamed-lengths
I'm not aware of any way to set this globally. Well, the first one can be set once. You'll still have to use "[" and "]" to force beaming, and you should, for the sake of Best Practices™, set the stemlet length manually for each one.
But this is the way I've had to do it; and while it's tedious to do that for every affected beam, it ensures that they look good and work properly.
Cheers,
A