Though this message was meant for Lasse, I’ll give a few thoughts. Perhaps you want to Private Message Lasse - either through the forum or by email. Oh, and perhaps make your modified module available so that people can take a look and give suggestions.
Chances are, that most of what you do will be forward-compatible - at least until VASSAL 4 is a reality 
To increase the probability of forward compatibility, reduce the amount of Java custom code - if any. Instead, try to use VASSAL traits, elements, BeanShell, and so on. If you do use Java custom code, make the source public so that others may do the actual rebuild - for example by including the code in the module directly, or via some public (Git) repository.
Are you sure you need to write actual source code? In many cases, it is absolutely not needed. See also Tools for module developers, and Creating Custom Classes. The simplest developer tool? VI
Yours,
Christian