All you have to do is to edit the default formula and customize it for your purpose, which is being keg-only. It is actually very easy to make your own customized version of formulas, and once you edit a formula, Homebrew keeps your personalized version for future uses.
So, just edit the formula file (for the specific tap version you intend to install) and add
keg_only "My reason for making this keg-only" within the formula class, for example on a new line directly after the
In case you haven't edited a formula before, read these few lines from Homebrew's FAQ in order to learn how to do so:
Can I edit formulae myself?
Yes! It’s easy! Just
brew edit $FORMULA.
You don’t have to submit modifications back to Homebrew/homebrew,
just edit the formula as you personally need it and
As a bonus
brew update will merge your changes with upstream
so you can still keep the formula up-to-date
with your personal modifications!