I suspect no one answered earlier because there
doesn't seem to be an easy way to do this. There
should be.
The brew-chord-bracket routine uses the horizontal,
bracket stencil, which does have a means of modifying the
end heights when used as a horizontal bracket, but this
does not seem to have been included in brew-chord-bracket.
(I hope someone can correct me if this is wrong)
You might try starting with a horizontal bracket of the
right shape, rotating it, and translating it to where
you want it with 'extra-offset, but this is really messy.
Not recommended unless you really need it and have only
one or two to do.
Trevor