That sounds disconcerting and distracting; I don't want my prompt to change while I'm typing a command. If I already have a command prompt, I can type "git status" easily enough.
Ah, I see. Interesting zsh magic, and the right-prompt mechanism makes it more palatable. Two issues, though. First, I use bash. Second, and more importantly, you're using the same temporary file for all shells, so the prompts from different shells (in different working directories) will overwrite each other.
Yeah, I thought about using a different temp file per shell, and I did use that for some time, but that got annoying when shells didn't exit gracefully and clean up the temp file.
I don't actually care about race conditions (okay, the wrong prompt may be displayed once, big deal), and it doesn't actually happen in real use because of the way timing works out.
Also, how would that work?