[Big-Foot]

Mike - I've not heard of the injector cap falling in like that before. The cap is usually bigger than the port due to it's rim. Sort of looks like a Derby-Hat.
Anyways - a couple of things.
If it happened on one, it may well happen on the others.

Your idea of lowering the valve might work better by removing the spring, then carefully lowering it and if you can see it from the spark-plug hole, you might be able to get in there with a micro-tip butane torche to burn the plastic into ashes..