如何实现自定义结构标记并能够对地图进行XML编码？

I have the following:

https://play.golang.org/p/ADX6H-bh0CU

package main

import (
    "encoding/xml"
    "fmt"
)

type xmlMap map[string]string

type xmlMapEntry struct {
    XMLName xml.Name
    Value   string `xml:",chardata"`
}

func (m xmlMap) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
    if len(m) == 0 {
        return nil
    }

    err := e.EncodeToken(start)
    if err != nil {
        return err
    }

    for k, v := range m {
        e.Encode(xmlMapEntry{XMLName: xml.Name{Local: k}, Value: v})
    }

    return e.EncodeToken(start.End())
}

func main() {
    type Family struct {
        Siblings map[string]string
    }

    type MyFamily struct {
        Siblings xmlMap `json:"siblings" xml:"siblings"`
    } 

    // In reality, "f" comes from a function in an outside package...e.g. "f := somepackage.Function()"
    f := &Family{
        Siblings: map[string]string{"bob": "brother", "mary": "sister"},
    }

    var m = &MyFamily{}
    *m = MyFamily(*f)

    x, err := xml.Marshal(m)
    if err != nil {
        panic(err)
    }
    fmt.Println(string(x))
}

The Family struct comes from an outside package. I created MyFamily struct in order to add json and xml tags. If I try to xml encode MyFamily then I get an error:

xml: unsupported type: map[string]string

This is easily resolved by implementing the custom XML marshaller that I have done.

This creates a separate problem. I have tried to simplify the example but "f" comes from a function in an outside package...e.g. "f := somepackage.Function()". When I try to Alias f to m I get the error:

cannot convert *f (type Family) to type MyFamily

This happens because Family has a map[string]string but MyFamily has xmlMap. Even though they are the same underlying type I get that error.

The question is, how do I implement my custom struct tags AND be able to XML encode a map?

EDIT: This is a simple example and can just be resolved by

var m = &MyFamily{Siblings: f.Siblings} 

Since there are MANY fields in both Family and MyFamily this is just inefficient and just wanted to know if there's a better way.