I authored a Java freeware (closed source) product that I deploy on a web host and distribute via JNLP, inclusive Linux clients.
I plan to suggest this product for inclusion into several Linux distro, if possible "as is" (JNLP-based).
Can I already contact distros, or I need to reconfigure something (deploy on another host, convert JNLP to something, etc.)? What is your experience?
Generally Linux distros aren't very interested in things they can't integrate into their release schedules. Since JNLP allows you to deploy a new version at any time, they can't do a version freeze before release. This will exclude you from most distros - Debian even removed Adobe's flash plugin for this very reason, and if they'll remove Flash, don't expect them to make exceptions for a less well known application.
If you're willing to distribute it as just a plain .jar, you might be able to get it in - for debian, you'll want to read the Debian New Maintainer's Guide and Debian Policy on how to package things, submit an ITP (intent to package), upload a package, then seek sponsorship on the debian-mentors mailing list. More info can be found at the Debian mentors FAQ. Packages submitted to Debian will also make their way into Ubuntu in due course.
Other distributions will have different policies, of course; if in doubt, check their faqs, or ask on an appropriate mailing list for the distro in question.
Java programs tend to be very difficult to package and maintain. Therefore guidelines vary from distro to distro. So you should have a look at the specific guidelines for some popular distros first.
In addition to that: Everything that updates itself independently from the package manager can be real pain for the user and maintainer of the package so you should consider a different distribution model for inclusion in linux distros.
I would say that a freeware JNLP application is pretty much the opposite of what Linux distributions would tend to include.
Firstly, JNLP will not work with the native package management solution. If you wanted to get an application included in a distribution it would need to be package in the native format and updated in the standard way.
Secondly, most distributions will favour open source packages and many will not include non open source packages in their default repositories. Some distributions may have specical non-free repositories. Up until OpenJDK you may not even Java itself would be in these repositories.
In my opinion you would be better trying to build a user base on your own as you then have complete control over releases etc.
Figure out a way to package it properly on each of the platforms you want to include your software on, and prepare for rigid software management (freezing, multiple versions etc if you want the code included in the standard installation.
Otherwise you may be able to create a simple package just containing the link to the JNLP page which may be included in the non-core parts. It is worth a try.
What applcation are we talking about?