Hi Guillermo L,
I usually don't recommend using script tasks at all unless you are doing a proof of concept or something really simple. It's is usually a bad practice to embed java code inside an xml file, but the bpmn2 specification allows it.
In jBPM6 we are making a huge effort to support CDI in all the framework, but in jBPM5.4 you can use it as well, you will need to do more work for sure. About service tasks, it shoudn't be so hard you can just implement a WorkItemHandler that access the CDI container to get the bean that you want.
Cheers