Run your rsync in one pane, with a filtered view in the second:
<span style="color:#323232;">tmux new 'rsync ...options... |& tee logview' ; split-window -h 'grep "denied" logview'
</span>
Replace …options… with your normal rsync command line.
That should give you a split view, with all the normal messages on the left, and only messages containing ‘denied’ on the right.
The |& makes sure we capture both stdout and stderr, tee then writes them to the fifo and displays them. split-window tells tmux to create a second pane, and display the output of grep.