Can the NFC hardware within Lumia 920 emulate a 125 kHz Proximity Access Card?
It looks like the NFC hardware implements the standard that is a superset of the standard that access cards use. But I don't have enough knowledge of those radio standards to understand if a phone can work only as a receiver or also as a transmitter of such signals.
I will also appreciate a link to a good overview article that explains those standards in simpler terms than the official specifications.
It is impossible from the physics perspective and standards. NFC is based upon ISO 14443 and is using 13,56 MHz wave carrier - which is high-frequency layer - it is typically the electric field scope.
125 kHz is the proximity scope (125 kHz - 134 kHz) which uses magnetic induction (and magnetic field) as the communication environment.
Conclusion - it is physically impossible in this exact case. You could if you would use 13,56-solutions as proximity (this is possible).
Take a look here: This white paper details the features in Windows Phone 8.1 that you can use to create a UICC-based NFC card-emulation app.
http://www.microsoft.com/en-us/download/details.aspx?id=43681
