There are lots of places where expressions cannot be used. An issue for this is in the jira. Please try to locate it and add this to it.
I used the EnvironmentImpl.getFromCurrent method to retrieve the referenced beans instead of using autowired fields. This way, I don't need to define the assignment handler as a bean in my spring container.
Is this acceptable as well?
I (personally) have no clue. Have not used spring a lot in general and not at all with jBPM. Maybe someone else has a clue.
According to me this approach is fine. Infact the purpose of Spring context class seems to be something like this. So my vote is for your solution.