Just in case anyone's wondering - the Board didn't discuss this issue at much length at the weekend, but the issue is certainly one to think about, and we asked Katherine to look into the subject.
I haven't yet mentioned my personal views based on my own experience. so here they are;
Personally I think the most sensible safeguards against 'entryism' is having a large and well-involved membership.
Another step some organisations take is to say that someone has to be a member for a certain length of time before conferring voting rights on them, though the only time I've seen this is enacted is when there have been serious problems with people joining to push particular agendas. (Also worth nothing that this in our case would need an amendment to the Articles.)
I'd also note that I've worked for several membership organisations and am a member of many more - none that I know of take any steps to verify memberships, even where someone is formally required to approve each member before admitting them. (And under what circumstances would we refuse membership? Not being on the electoral role doesn't mean someone doesn't exist...)
If we did do anything like this we would have to carefully consider whether the benefit in terms of preventing a potential, if somewhat hypothetical, democratic problem was actually bigger than any likely cost in terms of making it more difficult to join.