I'm running into something similar - I wanted to see what fraction of my income was going to taxes. It's not pretty, but this works:
ledger -f *.ledger bal --no-total expenses:tax income:earned --current --collapse 'abs(total)' --format '%(account) %(percent(display_total, -parent.total + display_total))\n' | sed -n 's/^expenses/tax overhead/p'
This just gets me:
tax overhead nn.nn%
I'd love a better way to do this, though. It tries to divide by zero with --depth 2, and doesn't easily scale to more than just one output value.