To me, to impose a restriction on characters from joining invasions (or, rather, joining a faction to partake in an invasion) to an arbitrary date prior to the start of the IC thread is absolutely backwards. The website allows us to use submissions without approval in any thread started after the submission is posted into the factory or codex, why would this not be the same for characters/writers? We're on a medium for cooperative writing, regardless of the fact that invasions are inherently competitive, it is still going a step too far to deny access to invasions for people who join a faction before the start of the thread but after the announcement that an invasion is going to occur.
I'm not going to weigh in on fairness and all of that, but I do prefer consistency in rules, and the only consistency that would be found is to either keep things as they are, with no limits (the direction of restrictions in the factory and codex regarding when submissions can be used in threads/invasions) or the lowest limits possible (before the start of the IC thread).
Are people going to "join" a faction to partake in an invasion? Yes, some will. Can we prevent this? Can you prevent me from joining every major faction right now, voiding this entire discussion?