I don't have any solution to offer, but I would like to second the wish for this to work better, and for the same reason: I have written several very useful functions for rewriting the command line, binding them with bind -x. Like Rob, I have concluded that it does not work well with a multi-line prompt, as the visual effect of the prompt moving further and further down is rather unpleasant.
I have for now accepted that I have to live with a one-line prompt, if I have to choose between my functions and a two-line prompt. Furthermore, I found this thread from 2009: https://lists.gnu.org/archive/html/bug-bash/2009-07/msg00075.html where Chet talks about the possibility of a function requesting that the prompt not be redrawn at all, and I would love to see this implemented. There are many great things one can do with the "bind -x" command, but some of them are a little less attractive when they cause a redrawing of the prompt. Jesper > >
