Zsh right-justify in ps1

You have to include

setopt PROMPT_SUBST

in your .zshrc, man zshall explains it in the PROMPT EXPANSION section:

If the PROMPT_SUBST option is set, the prompt string is first subjected to parameter expansion, command substitution and arithmetic expansion.

The simplest approach to emulate bash's $PROMPT_COMMAND which comes to my mind is to use the precmd hook, as you already figured out. Define it as

precmd() { eval "$PROMPT_COMMAND" }

and you can do something like that:

$ PROMPT_COMMAND='echo Hello, it is now $(date)'
Hello, it is now Mon, Mar 31, 2014 7:08:00 PM
$ whoami      
user
Hello, it is now Mon, Mar 31, 2014 7:08:21 PM     
$

Please note the single quotes in that example, otherwise $(date) will get expanded too early, i.e. already when defining $PROMPT_COMMAND and not when called before the prompt.

If you want to preserve (and don't want to alter) the existing definition, you can use that approach:

$ prmptcmd() { eval "$PROMPT_COMMAND" }
$ precmd_functions=(prmptcmd)

With that the prmptcmd functions is executed after the existing precmd() function.

Finally, here is a way which is suitable for use in a program package, which neither should modify user or system files nor can enter the commands interactive.

An example to spawn a bash session could be

PROMPT_COMMAND="echo foo" bash

To spawn zsh you can use

ZDOTDIR=/program/dir zsh

which causes /program/dir/.zshrc to be sourced. In this file the precmd() hook can be defined as explained above. If you want the user's settings in addition include source $HOME/.zshrc etc. in the program's .zshrc, too. This setup is maintainable, as no files outside the program directory are modified.

As a last addition, here is a proof of concept how to keep the newuser welcome, too. Use the following code in your /program/dir/.zshenv rc config file:

echo define precmd, traps, etc.

autoload -Uz zsh-newuser-install

if [[ ! -e "$HOME/.zshrc" ]]; then
  zsh-newuser-install -f
  mv $ZDOTDIR/.zshrc $HOME/.zshrc
else
  builtin source $HOME/.zshrc
fi