I gave in and installed bash from Homebrew.
Realize that if your personal account's shell is pointed into the homebrew installation in /usr/local, and you implode homebrew or wipe /usr/local, you won't be able to start a terminal until your shell is changed or the binary exists. Solving this without Terminal.app being able to launch is a minor annoyance.
#!/bin/sh
for shell in \
/opt/local/bin/bash \
/bin/bash \
/bin/sh
do
[ -x "$shell" ] && exec "$shell" -l
done
echo >&2 "ERROR: No valid shell found!"
read -r -s -n 1 -t 10 dummy
exit 1