bbPress.org

Yeah, you could copy the wp_register() function into your functions.php file, modify it (and its name), and then use the modified function instead.

Actually it should be

global $id;

Sorry. And for the last line, you have the right idea but you have to use the string concatenation (putting together into one) operator: the dot . This part is gonna be a bit more complicated, but solution at the bottom.

Strings are indicated by using apostrophes ' ... ' or quotation marks " ... ". If you start a string with one of them, the other one doesn’t affect anything. Variable names inside the quotation marks " will be replaced with their value, but inside ' they will not. The strings here are made using apostrophes ', so you can have valid HTML code without problems: '<a href="..."' works, but "<a href="..."" doesn’t. But that means variable names inside the string won’t get replaced with their value. So you have to concatenate the strings, like this: '<a href="...' . $id . '"> ...';

$link = $before . '<a href="' . get_option('siteurl') . '/bbpress/profile.php?id=' . $id . '">' . __('Profile') . '</a>' . $after;

Should work. Bit tired and therefore also inclined to ramble; sorry about that.