It's not exactly what you're looking for but it is pretty close. The idea is to bind a key to a set of commands to:
- Prompt for the command to run
- Use 'select-pane' to chose the first top level pane
- Use 'send-keys' to run the command
- Repeat steps 2 and 3 for other top level panes
Here is how the command looks like
bind R command-prompt -p "Command :" "select-pane -t 0 \; send-keys "%1" C-m \; select-pane -t 2 \; send-keys "%1" C-m \; select-pane -t 4 \; send-keys "%1" C-m "
Following is a complete example, let's setup the 6 panes, 2 rows of 3 each :
$ tmux new -s 'top_n_tail' \; split-window -h \; split-window -h \; select-layout even-horizontal \; detach
$ tmux att -t 'top_n_tail' \; select-pane -t 0 \; split-window \; detach
$ tmux att -t 'top_n_tail' \; select-pane -t 2 \; split-window \; detach
$ tmux att -t 'top_n_tail' \; select-pane -t 4 \; split-window
On tmux prompt (C-b :) bind 'R' to a set of tmux commands which accept your bash command and send it to some panes:
:bind R command-prompt -p "Command :" "select-pane -t 0 \; send-keys "%1" C-m \; select-pane -t 2 \; send-keys "%1" C-m \; select-pane -t 4 \; send-keys "%1" C-m "
Now when you hit C-b R, you'll be prompted for a command

Which will run only in the top 3 panes