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Wow, tis is a blast from the past
. I think your somewhere between a rock and a hard place with this Andy as the GPL which XOOPS is released under is quite clear on this. If you are to make a living out of developing for and supporting Xoops, then the only options I can see that protect your interests are;
(a) Never actually release your module code, but instead just allow your customers exclusive access to be able to use it on their website (the same as your option b above). Make sure that you make it very clear for customers which modules are available for use exclusively through your hosting services.
(b) Expand your modules so that they are actually able to run as a stand alone item, and independently of Xoops. This will release you from being tied into the GPL, and will then allow you to release your code under any license you wish.
Option b would be a lot of extra work, and I doubt that what you would gain by doing it would be worth your while. This leaves you with option a, which as you say is not exactly fair, but is the only one that protects your interests as a module developer for a GPL product. There is not a thing you can do to stop people from distributing your code that you depend on for your income once it is released, so the only card you as a developer have left to play is to not release the code at all and allow people the use of it in a controlled hosting environment.