Afaik, there is no such ootb mechanism.
It's either a module or embedded.
But you could easily add this programatically -- via custom subsystem.
e.g. Seam2 support is done in such a way
is there any clue how to do this?
For now I managed to register a custom subsystem based on custom server module. How to set applications now to use resources of this subsystem?
You could split your common.jar in two jars
that would be one way of doing it.
other would be simply define two modules that use same common.jar and then use include / exclude paths for some packages where you have your singleton classes.
in any case is see that it would be better to split your jar in two parts, one with singletons, other with everything else.