Re: Redirecting the bp_login_bar – almost got it!
You are talking about the function bp_login_bar() in the bp-core-templatetags.php file.
The form field that you are interested in gets set by bp before it arrives at the user’s browser. You don’t know who the user is until they fill out the form and it gets sent back to the server for processing.
Then they are logged in and you know who they are. It’s too late though because the redirect takes effect after login and they wind up at wherever the hidden field redirect sends them to.
You’ll have to investigate something like this: