The issue with the delegateExpression is that it MAY resolve to another bean every single time it's called, so field-injection can't be done just once, as it is with the normal JavaDelegates. However, you get the execution passed in, you can extract all variables you want but I understand the fact that you code the expressions, rather than define them in
your process file.
AFAIK, nothing is planned for this. As I see delegateExpressions, is to have the resolving the of the javaDelegate fully controlled in the developer's hands. Having field-injection on that as well is a valid use case, although not crucial.
You could always suggest a solution based on the current functionality or propose a patch, we're happy to accept contributions in this area.