The inputs are always available by reading the state of the input port at any time. Therefore, if the application does not catch the transition messages, a periodic poll (refer to command set document) would do the trick.
For one, I have seen the EZIO return 0xFF when polling the input status. Also, For a security application, you would need to poll at a rate much faster than a powerline is capable of sustaining (unless the inputs are latched). I am currently polling at once per minute which is fine for a garage door, but for an entry door someone could easily open and then shut the door within 1 minute. Granted it would be rare, but we are talking about a security application! I would not be comfortable installing a security system that only works most of the time!
If you can not rely on the broadcast messages from the EZIOxx, you should be able to link a responder device that simply takes the group commands from the EZIOxx and passes them to an application.
Also, I do not want to have to go through the multiple button presses to link every input to some other responder. I could concievably look into automating this, but I would still need to have an extra Insteon responder. Would another PLM work? I would always have anouther PLM for HomeSeer to interface to the powerline.