Is it possible for an Android application to send a previously stored RFID card ID to a NFC reader? The aim would be to use an Android device to enable access to a room instead of a physical RFID card. Having read a lot of other threads about NFC/RFID & card-emulation, I came to the conclusion it was not available for now in Android but I'm still kinda confused if it apply to this particular case. Can't the application simply send the card ID within a NDEF message or an APDU command or I'm just completely dreaming?
Well, strictly speaking. For what you want to do you don't need card emulation. You just need to send a token to the door lock that can be validated.
You could do this with Android beam by pushing an NDEF message to a device that is compatible, SNEP is the protocol you'll be looking for.
A solution I would prefer would be to get the door lock device to emulate a tag. Then you could have your Android app register a listener for that tag (Doesn't even need to be running). When the tag is detected that app will fire up and send your secure token to the lock by using the tag write NFC functions in the Android SDK.
Securing your token is another matter.