Hi Peter,
I don't know why there is a limit. Maybe performance wise if you have super long strings that it might take super long, or takes up more memory.
If you want to keep the limit I think you'd have to create a new entity with unlimited attributes in some sort of CommunityCommons_Custom module and copy the java action in there as well. You'd have to alter the java action to then use that newly created entity as the variable to write it to, and don't forget to set it as the return variable as well.
Hope this helps!
KR, Jurre