How can I access all msg fields recursively in a message_generator?

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You might not need to iterate the msg fields recursively in the first place. It should be sufficient to iterate only the fields of the current message and for non-builtin field type invoke the serialization / deserialization function of the sub message (rather then implement it again within this message). At least that is the way gencpp, genpy and genlisp deal with it.

Update:

Pointing you to code might not be the best help since the message generator code / templates are everything but easy to read. You could look at the gencpp and genpy packages though.

What I am referring to is the following idea: for each built-in type you will have e.g. a serialize function:

serialize(bool)
serialize(int)

When you now want to serialize a Message Foo you will serialize all of its members like this:

serialize(Foo) {
    serialize(my_bool_member)
    serialize(my_int_member)
}

For a nested message you also only care about its member and not about their recursive fields because you will delegate the serialization of sub messages to there existing serialize implementation:

serialize(Bar) {
    serialize(my_foo_member)
    serialize(my_other_foo_member)
}

I'm not exactly sure what you're trying to accomplish but I'd like to propose to take a look at the built-in "dir()" function in python. For example:

>>> from sensor_msgs.msg import LaserScan
>>> dir(LaserScan)
['__class__', '__delattr__', '__doc__', '__eq__', '__format__', '__getattribute__', '__getstate__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__setstate__', '__sizeof__', '__slots__', '__str__', '__subclasshook__', '_check_types', '_connection_header', '_full_text', '_get_types', '_has_header', '_md5sum', '_slot_types', '_type', 'angle_increment', 'angle_max', 'angle_min', 'deserialize', 'deserialize_numpy', 'header', 'intensities', 'range_max', 'range_min', 'ranges', 'scan_time', 'serialize', 'serialize_numpy', 'time_increment']

This would give you the ability to "access all message fields recursively".