Not targeting a specific toolbar set, but we've implemented this for Newsletters - might help point you in the right direction for BizForms:
config.enterMode = location.pathname.indexOf('/Newsletters/Newsletter_Iframe_Edit.aspx') > -1 ? CKEDITOR.ENTER_BR : CKEDITOR.ENTER_P;
Would be nice if there was a way to set the enterMode per toolbar, though.