I’ve always been looking for a way in NX/FreeNX to be able to authenticate using mechanisms other than username and password, like SSH private/public keys or Kerberos. Turns out that it is possible
Someone pointed me to the FreeNX 0.7.3 announcement that contains the following excerpt:

Usermode and SUID Wrapper
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We are now very close to [...]

Introduction
DISCLAIMER: The contents of this post are mostly based on Manual Installation How-To. Thanks to Brent Davidson and Fabian Franz for writing such a nice HowTo and the beautiful open and free implementation of FreeNX, respectively.
I decided to use FreeNX instead of NoMachine’s own implementation due to the instability of the latter. Most of the [...]

FreeNX is based on NoMachine.com NX compression GPL components to allow a fast, graphical remote desktop terminal session for UNIX-based systems. NX uses SSH tunneling to perform authentication and link parameters negotiation.
NoMachine.com has NX viewer clients for Mac OS X and Linux. By default, the nxclient software uses a built-in private key to allow connecting [...]