Has anyone contacted BALPA to see how many are in fact members?
Also, In my airline one of my collegues publishes an unofficial crewlist with everyones email and phone number. When we tried to establish how many people were union members 1 individual send out an email to all requesting membership status. One way to find out how many you've got/ how many you need. Perhaps a crewlist is a good start. One could even set up an psuedo email if repurcussion is a concern.
Just a thought.
PF