Hi Michael,
I don't think this is a solution. Well technically it is, but I don't want to start creating mail nodes to manage my notification.
I actually found out in the meantime that what I would like to do is achievable with a reminder instead of a notification, by setting a very short first reminder, e.g. 15 seconds. It's kind of silly as it doesn't allow me to specify different messages for a notification and a reminder, but I could live with it:
<task candidate-groups="managers" g="243,49,127,52" name="Review Registration">
<reminder duedate="15 seconds" repeat="2 days" template="jbpm_approvepartreg_task" />
<transition name="approve" to="State 1" g="7,-11" />
<transition name="reject" to="State 2" g="-13,8" />
</task>
If nobody has a better solution I will use this one. Thanks for your answer anyway.
Cheers,
Andrea