Output is buffered, so piping to a command like grep has to have the pipe buffer filled before outputting ot the shell.
As a work around you can do this in the mean time (the real solution is to prevent output buffering in the app)
brew install homebrew/dupes/expect
unbuffer ./livetail ' "homepage" _sourceCategory=access_log' | grep " 200 "
Output is buffered, so piping to a command like grep has to have the pipe buffer filled before outputting ot the shell.
As a work around you can do this in the mean time (the real solution is to prevent output buffering in the app)