in reply to Append a tools status to the command line..
Changing the command line usually involves changing an environment variable in your shell. Since processes can't change each others' environment variables directly, you can't change a prompt from another process.
What I'd recommend is having the tool wrapper write a status line to a file, then using shell-escapes to insert a `cat /whatever/the/file/is` into your prompt. See your shell documentation for details.