Simple really is better, more times than not.
Yesterday I figured out how to track savings goals in Beancount (Fava) by creating a parallel BGT currency. It involves more data entry, but it does the calculation between what's already saved and my goal.
Nice, but unnecessary.
Today I scrapped all that (yay for backups!) and simply added the savings goal to the name of each account. Thanks for the tip on the forum, @simonmic