for the relative part, we already have:

https://github.com/zopefoundation/Zope/blob/d16ffc26d2d9aa3657ebad053ff9085e71f86984/src/OFS/Traversable.py#L79

that returns the relative path and seems to support more use cases. The only difference is the / prefix but being from the portal, it does not change the meaning, you can just use absolute_url_path and remove the /.

Note: I think that having the / prefix is better because this is an 'absolute-path reference':
https://github.com/zopefoundation/Zope/blob/d16ffc26d2d9aa3657ebad053ff9085e71f86984/src/OFS/Traversable.py#L80-L84

        """Return the path portion of the absolute URL of the object.

        This includes the leading slash, and can be used as an
        'absolute-path reference' as defined in RFC 2396.
        """

but this is not important.