Is anything being done yet for employers to be able to furlough employees who are new starters in March?

Unfortunately not. This was brought up to the Chancellor, Rishi Sunak, last week, and the suggestion from the government is to contact your previous employer and ask to be rehired and furloughed through them, and then let go at the end of the period. This is not an option for many employees, as quite often employers are not that happy when an employee moves on, so this is not the best solution.