I'm sure this has already been answered elsewhere, but I'm in the same boat and I believe the below link takes you to the answer.   https://www.cisco.com/c/en/us/products/collateral/routers/4000-series-integrated-services-routers-isr/data_sheet_c78-7...

I just tested this in my environment and if the WOL server sends out a directed broadcast there is no need for the helper addresses on the Wake On LAN server environment.  The only thing you need to do is enable ip directed-broadcast oon the client...