I think the point about a "show of hands" is that the union reps running the show on the day can sway the vote in any way they want. The "is there anybody brave enough to vote No" type comments can be asked first to gain a Yes vote, or the opposite "is anyone silly enough to vote Yes?" The point is that the show of hands is essentially an exercise in the dynamics of large groups and is open to manipulation - it is not a democratic process.
The people that are likely to turn up at such meetings are those that are politically aware and active. These are the same ones that are likely to vote No, as they are the same group that is invested in maintaining the power of BASSA. Historically this has been an entirely sensible decision, it has delivered enviable T&Cs, so maintaining the power of the union is not (in isolation) an unreasonable stand point.
So even if the offer is reasonable and would be acceptable to the majority of BASSA members, it is entirely likely that the whole meeting on the 12th will be used to prolong a dispute that most just want resolved. Essentially the issue is that a few politically active union members will manipulate the situation to force their opinion on the majority (whose opinion on the offer we will not be allowed to know as there will be no democratic vote).