Sourcing .zshrc works reasonably well. I have the following at the end of setup.sh -- which creates symlinks and sets up other configurations.
makeLinks does most of the work, then sets Homebrew's zsh as the default shell -- this currently runs in bash, so it probably will need to be updated at some point -- and everything gets reloaded.
makeLinks && chsh -s /usr/local/bin/zsh
exec $SHELL && brew doctor
makeLinks does most of the work, then sets Homebrew's zsh as the default shell -- this currently runs in bash, so it probably will need to be updated at some point -- and everything gets reloaded.