The --profile option is passed to each individual command, so technically you are not "IN" a profile until the command is running. For that reason, I don't think it's possible to do exactly what you're trying to do using just the --profile switch.
However, if you use the AWS_DEFAULT_PROFILE environment variable, it is possible.
Here's what I do in .bashrc
:
function startaws {
export AWS_DEFAULT_PROFILE=$1
}
(You can name the function whatever you like, of course).
This allows you to do something like startaws dev
, which both loads your "dev" profile, AND gives you the $AWS_DEFAULT_PROFILE environment variable, based on which you can now change your bash prompt.