You probably want to set the window option remain-on-exit to true for that window. I do not use tmuxinator, so I do not know whether/how this can be cleanly configured in that context. Here is how you would use a plain tmux command to set it for an existing window named worker:
tmux set-option -w -t :worker remain-on-exit on
This is a window option, so it will apply to all the panes in that window. After a pane’s command has exited, you can use respawn-pane to start its command anew.
tmux respawn-pane -t :worker.0
(Or Prefix:respawn-pane to respawn the current pane.)
This may not do what you expect, though, since tmuxinator seems to always create “default” panes/windows and then “type in” your configured command (i.e. the command restarted by respawn-pane will be a plain shell, not the command configured for the pane in your tmuxinator configuration). You may want to run your command in a loop instead (in your tmuxinator configuration):
while :; do QUEUE=high,normal,low rake resque:work; printf 'Hit enter to re-run... (C-c to abort)'; read; done