You could use a newline in PS1 and have the current directory above the prompt. It's not the same as RPROMPT, but I suppose it solves the problem.
But then again, I don't think you'll really "use bash when bash learns RPROMPT", simply because there is no reason to use Bash if you're happy with Zsh[1].
[1] I don't want to start a shell wars thread, but I'm curious if Bash has any advantages over Zsh other than ubiquity and the user being familiar with Readline (has a customized .inputrc for example).