Got my questions answerd by cmb (thanks again!) on the support mailing list. Here they are for the archives and anyone else searching the forums:
For the FTP helper to be started on the WAN interface, you need have the FTP helper enabled for that interface, a NAT rule for server port 21 defined and if not NATing the WAN IP, be using a CARP Virtual IP address (not ProxyARP or Other).
Anything can be entered for the CARP VIP password, group and frequency.
The FTP helper is started by code in /etc/inc/filter.inc.