OK, found a workaround.
If you make the change Jason suggested above, you must also set the fedex service to GROUNDHOMEDELIVERY or else it throws an error.
Also, please note there are 3 instances of $FedEx_XML .= "<v10:Residential>0</v10:Residential>"; in that file, 2 of which have to do with the shipper address, which you dont want to change.
However, if you give the user a choice to choose residential or business address, the logic is easy to set fedexService to either GROUNDHOMEDELIVERY or FEDEXGROUND, but there is no easy way to change the logic in WA-eCart_FedEx_PHP.php to be either residential (1) or not (0).
Any chance of an update here? Or at least incorporating that into eCart 6?