JNLP still doesn't provide the ability to query for license keys, at least not explicitly.
What it does provide so far is the ability of downloading only the difference of JAR files.
Say you have an app that is composed of three JARs: JAR1-3. Then you post on your deploymen server a new version, that solves some bugs and the JAR1 need to be downloaded again with the newer version. If your deployment server can run servlets, you can use a JNLP server implementation that supports the JARDiff protocol.
At this point, say that the old JAR1 takes 850KB. The new one (JAR1a) is pretty the same as the old, just 1KB changes (it is actually a patch release). If your server supports JARDiff, then your clients will download only 1KB, not the whole JAR1a anew.
This is an example of an "advanced" service offered by a JNLP Server. Anyway, if you don't need such a feature, you can happily use a static web server, as discussed in my book or in any JNLP documentation.
Cheers,
Mauro