Thank you for the suggestion! But, I'm sorry to say that the game I'm building may not have room to support it. In particular, there are already risk/reward systems in place to encourage getting close to bursting. So, in terms of game design, I don't think an "immobile" stage between "normal" and "popped" would add much.
In terms of kink design, though, lingering longer in that moment where a character (or the player) is helplessly doomed to pop at another's hand does sound enticing. I'm going to keep that in mind, even though an immobile stage per se probably isn't happening. Same goes for having characters comment on your current size, especially as you get huge. I might work that into the existing character dialogue system...